diff --git a/composer.json b/composer.json
index e5ec9a8..6920478 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,9 @@
     "ext-iconv": "*",
     "ext-ctype": "*",
     "psr/log": "^1.0|^2.0|^3.0",
-    "doctrine/lexer": "^3.0"
+    "doctrine/lexer": "^3.0",
+    "symfony/mime": "^7.0",
+    "egulias/email-validator": "^4.0"
   },
   "require-dev": {
     "phpunit/phpunit": "^11.0"
@@ -43,4 +45,4 @@
       "test:unit": "phpunit --configuration tests/php/phpunit.unit.xml --colors=always --testdox",
       "test:coverage": "XDEBUG_MODE=coverage phpunit --configuration tests/php/phpunit.unit.xml --coverage-html .phpunit.coverage --coverage-text"
   }
-}
\ No newline at end of file
+}
diff --git a/composer.lock b/composer.lock
index a45a0f4..b95625e 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "46ea36f63cc6f77b99348c76fc0f72b2",
+    "content-hash": "63573b43a17b583b8766d74720f367a3",
     "packages": [
         {
             "name": "doctrine/lexer",
@@ -83,6 +83,73 @@
             ],
             "time": "2024-02-05T11:56:58+00:00"
         },
+        {
+            "name": "egulias/email-validator",
+            "version": "4.0.4",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/egulias/EmailValidator.git",
+                "reference": "d42c8731f0624ad6bdc8d3e5e9a4524f68801cfa"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/egulias/EmailValidator/zipball/d42c8731f0624ad6bdc8d3e5e9a4524f68801cfa",
+                "reference": "d42c8731f0624ad6bdc8d3e5e9a4524f68801cfa",
+                "shasum": ""
+            },
+            "require": {
+                "doctrine/lexer": "^2.0 || ^3.0",
+                "php": ">=8.1",
+                "symfony/polyfill-intl-idn": "^1.26"
+            },
+            "require-dev": {
+                "phpunit/phpunit": "^10.2",
+                "vimeo/psalm": "^5.12"
+            },
+            "suggest": {
+                "ext-intl": "PHP Internationalization Libraries are required to use the SpoofChecking validation"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "4.0.x-dev"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "Egulias\\EmailValidator\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Eduardo Gulias Davis"
+                }
+            ],
+            "description": "A library for validating emails against several RFCs",
+            "homepage": "https://github.com/egulias/EmailValidator",
+            "keywords": [
+                "email",
+                "emailvalidation",
+                "emailvalidator",
+                "validation",
+                "validator"
+            ],
+            "support": {
+                "issues": "https://github.com/egulias/EmailValidator/issues",
+                "source": "https://github.com/egulias/EmailValidator/tree/4.0.4"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/egulias",
+                    "type": "github"
+                }
+            ],
+            "time": "2025-03-06T22:45:56+00:00"
+        },
         {
             "name": "psr/log",
             "version": "3.0.2",
@@ -132,6 +199,423 @@
                 "source": "https://github.com/php-fig/log/tree/3.0.2"
             },
             "time": "2024-09-11T13:17:53+00:00"
+        },
+        {
+            "name": "symfony/deprecation-contracts",
+            "version": "v3.7.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/deprecation-contracts.git",
+                "reference": "50f59d1f3ca46d41ac911f97a78626b6756af35b"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/deprecation-contracts/zipball/50f59d1f3ca46d41ac911f97a78626b6756af35b",
+                "reference": "50f59d1f3ca46d41ac911f97a78626b6756af35b",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.1"
+            },
+            "type": "library",
+            "extra": {
+                "thanks": {
+                    "url": "https://github.com/symfony/contracts",
+                    "name": "symfony/contracts"
+                },
+                "branch-alias": {
+                    "dev-main": "3.7-dev"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "function.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "A generic function and convention to trigger deprecation notices",
+            "homepage": "https://symfony.com",
+            "support": {
+                "source": "https://github.com/symfony/deprecation-contracts/tree/v3.7.0"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-04-13T15:52:40+00:00"
+        },
+        {
+            "name": "symfony/mime",
+            "version": "v7.4.13",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/mime.git",
+                "reference": "a845722765c4f6b2ce88beaf4f4479975b186770"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/mime/zipball/a845722765c4f6b2ce88beaf4f4479975b186770",
+                "reference": "a845722765c4f6b2ce88beaf4f4479975b186770",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.2",
+                "symfony/deprecation-contracts": "^2.5|^3",
+                "symfony/polyfill-intl-idn": "^1.10",
+                "symfony/polyfill-mbstring": "^1.0"
+            },
+            "conflict": {
+                "egulias/email-validator": "~3.0.0",
+                "phpdocumentor/reflection-docblock": "<5.2|>=7",
+                "phpdocumentor/type-resolver": "<1.5.1",
+                "symfony/mailer": "<6.4",
+                "symfony/serializer": "<6.4.3|>7.0,<7.0.3"
+            },
+            "require-dev": {
+                "egulias/email-validator": "^2.1.10|^3.1|^4",
+                "league/html-to-markdown": "^5.0",
+                "phpdocumentor/reflection-docblock": "^5.2|^6.0",
+                "symfony/dependency-injection": "^6.4|^7.0|^8.0",
+                "symfony/process": "^6.4|^7.0|^8.0",
+                "symfony/property-access": "^6.4|^7.0|^8.0",
+                "symfony/property-info": "^6.4|^7.0|^8.0",
+                "symfony/serializer": "^6.4.3|^7.0.3|^8.0"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Component\\Mime\\": ""
+                },
+                "exclude-from-classmap": [
+                    "/Tests/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Fabien Potencier",
+                    "email": "fabien@symfony.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Allows manipulating MIME messages",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "mime",
+                "mime-type"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/mime/tree/v7.4.13"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-05-23T16:22:37+00:00"
+        },
+        {
+            "name": "symfony/polyfill-intl-idn",
+            "version": "v1.38.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/polyfill-intl-idn.git",
+                "reference": "dc21118016c039a66235cf93d96b435ffb282412"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/polyfill-intl-idn/zipball/dc21118016c039a66235cf93d96b435ffb282412",
+                "reference": "dc21118016c039a66235cf93d96b435ffb282412",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.2",
+                "symfony/polyfill-intl-normalizer": "^1.10"
+            },
+            "suggest": {
+                "ext-intl": "For best performance"
+            },
+            "type": "library",
+            "extra": {
+                "thanks": {
+                    "url": "https://github.com/symfony/polyfill",
+                    "name": "symfony/polyfill"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ],
+                "psr-4": {
+                    "Symfony\\Polyfill\\Intl\\Idn\\": ""
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Laurent Bassin",
+                    "email": "laurent@bassin.info"
+                },
+                {
+                    "name": "Trevor Rowbotham",
+                    "email": "trevor.rowbotham@pm.me"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Symfony polyfill for intl's idn_to_ascii and idn_to_utf8 functions",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "compatibility",
+                "idn",
+                "intl",
+                "polyfill",
+                "portable",
+                "shim"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/polyfill-intl-idn/tree/v1.38.1"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-05-25T15:22:23+00:00"
+        },
+        {
+            "name": "symfony/polyfill-intl-normalizer",
+            "version": "v1.38.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/polyfill-intl-normalizer.git",
+                "reference": "2d446c214bdbe5b71bde5011b060a05fece3ae6b"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/polyfill-intl-normalizer/zipball/2d446c214bdbe5b71bde5011b060a05fece3ae6b",
+                "reference": "2d446c214bdbe5b71bde5011b060a05fece3ae6b",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.2"
+            },
+            "suggest": {
+                "ext-intl": "For best performance"
+            },
+            "type": "library",
+            "extra": {
+                "thanks": {
+                    "url": "https://github.com/symfony/polyfill",
+                    "name": "symfony/polyfill"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ],
+                "psr-4": {
+                    "Symfony\\Polyfill\\Intl\\Normalizer\\": ""
+                },
+                "classmap": [
+                    "Resources/stubs"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Symfony polyfill for intl's Normalizer class and related functions",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "compatibility",
+                "intl",
+                "normalizer",
+                "polyfill",
+                "portable",
+                "shim"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/polyfill-intl-normalizer/tree/v1.38.0"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-05-25T13:48:31+00:00"
+        },
+        {
+            "name": "symfony/polyfill-mbstring",
+            "version": "v1.38.2",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/polyfill-mbstring.git",
+                "reference": "d3d318bad5e7a1bfbd026009c8bfb8d8f99ae6b6"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/polyfill-mbstring/zipball/d3d318bad5e7a1bfbd026009c8bfb8d8f99ae6b6",
+                "reference": "d3d318bad5e7a1bfbd026009c8bfb8d8f99ae6b6",
+                "shasum": ""
+            },
+            "require": {
+                "ext-iconv": "*",
+                "php": ">=7.2"
+            },
+            "provide": {
+                "ext-mbstring": "*"
+            },
+            "suggest": {
+                "ext-mbstring": "For best performance"
+            },
+            "type": "library",
+            "extra": {
+                "thanks": {
+                    "url": "https://github.com/symfony/polyfill",
+                    "name": "symfony/polyfill"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ],
+                "psr-4": {
+                    "Symfony\\Polyfill\\Mbstring\\": ""
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Symfony polyfill for the Mbstring extension",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "compatibility",
+                "mbstring",
+                "polyfill",
+                "portable",
+                "shim"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/polyfill-mbstring/tree/v1.38.2"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-05-27T06:59:30+00:00"
         }
     ],
     "packages-dev": [
@@ -1919,7 +2403,7 @@
     ],
     "aliases": [],
     "minimum-stability": "stable",
-    "stability-flags": [],
+    "stability-flags": {},
     "prefer-stable": true,
     "prefer-lowest": false,
     "platform": {
@@ -1927,9 +2411,9 @@
         "ext-iconv": "*",
         "ext-ctype": "*"
     },
-    "platform-dev": [],
+    "platform-dev": {},
     "platform-overrides": {
         "php": "8.2"
     },
-    "plugin-api-version": "2.3.0"
+    "plugin-api-version": "2.6.0"
 }
diff --git a/lib/Mime/MessageBuilder.php b/lib/Mime/MessageBuilder.php
new file mode 100644
index 0000000..70f06b6
--- /dev/null
+++ b/lib/Mime/MessageBuilder.php
@@ -0,0 +1,271 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: Sebastian Krupinski <krupinski01@gmail.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace KTXM\ProviderImap\Mime;
+
+use DateTimeImmutable;
+use KTXF\Mail\Object\AddressInterface;
+use KTXF\Mail\Object\MessagePartInterface;
+use KTXF\Mail\Object\MessagePropertiesBaseInterface;
+use Symfony\Component\Mime\Address as MimeAddress;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Part\DataPart;
+
+/**
+ * MIME Message Builder
+ *
+ * Converts a framework {@see MessagePropertiesBaseInterface} into raw RFC822
+ * bytes. The output is transport-neutral and used by both of this module's
+ * outbound paths: the SMTP DATA payload (submission) and the IMAP APPEND
+ * literal (saving a copy to the Sent collection).
+ *
+ * The MIME engine (symfony/mime) is intentionally hidden behind this API so it
+ * can be swapped without touching consumers.
+ *
+ * @since 2026.06.22
+ */
+final class MessageBuilder
+{
+    /**
+     * Headers that are derived from structured properties and therefore must
+     * not be copied verbatim from {@see MessagePropertiesBaseInterface::getHeaders()}.
+     *
+     * @var list<string>
+     */
+    private const RESERVED_HEADERS = [
+        'from', 'sender', 'reply-to', 'to', 'cc', 'bcc', 'subject', 'date',
+        'message-id', 'mime-version', 'content-type', 'content-transfer-encoding',
+        'in-reply-to', 'references', 'bcc',
+    ];
+
+    /**
+     * Build the complete RFC822 message bytes for the given properties.
+     */
+    public function build(MessagePropertiesBaseInterface $message): string
+    {
+        return $this->toEmail($message)->toString();
+    }
+
+    /**
+     * Collect the envelope recipients (To + Cc + Bcc) as bare e-mail addresses.
+     *
+     * Useful for the SMTP RCPT TO phase, which needs every recipient including
+     * Bcc, even though Bcc never appears in the rendered headers.
+     *
+     * @return list<string>
+     */
+    public static function recipients(MessagePropertiesBaseInterface $message): array
+    {
+        $recipients = [];
+
+        foreach ([...$message->getTo(), ...$message->getCc(), ...$message->getBcc()] as $address) {
+            $email = trim($address->getAddress());
+            if ($email !== '' && !in_array($email, $recipients, true)) {
+                $recipients[] = $email;
+            }
+        }
+
+        return $recipients;
+    }
+
+    private function toEmail(MessagePropertiesBaseInterface $message): Email
+    {
+        $email = new Email();
+
+        if (($from = $message->getFrom()) !== null) {
+            $email->from($this->address($from));
+        }
+        if (($sender = $message->getSender()) !== null) {
+            $email->sender($this->address($sender));
+        }
+
+        $replyTo = $this->addresses($message->getReplyTo());
+        if ($replyTo !== []) {
+            $email->replyTo(...$replyTo);
+        }
+
+        $to = $this->addresses($message->getTo());
+        if ($to !== []) {
+            $email->to(...$to);
+        }
+
+        $cc = $this->addresses($message->getCc());
+        if ($cc !== []) {
+            $email->cc(...$cc);
+        }
+
+        $bcc = $this->addresses($message->getBcc());
+        if ($bcc !== []) {
+            $email->bcc(...$bcc);
+        }
+
+        $email->subject($message->getSubject());
+
+        if (($sent = $message->getSent()) !== null) {
+            $email->date($sent instanceof DateTimeImmutable ? $sent : DateTimeImmutable::createFromInterface($sent));
+        }
+
+        $text = $message->getBodyTextPlain();
+        $html = $message->getBodyTextHtml();
+        // Inline attachments may carry Content-IDs without an "@"; those must be
+        // normalised (symfony requires an "@") and the matching cid: references
+        // in the HTML body rewritten so the parts still resolve.
+        $cidRewrites = $this->inlineCidRewrites($message);
+        if ($text !== null && $text !== '') {
+            $email->text($text);
+        }
+        if ($html !== null && $html !== '') {
+            $email->html($this->rewriteCids($html, $cidRewrites));
+        }
+        // Guarantee at least an (empty) text part so the message is well-formed.
+        if (($text === null || $text === '') && ($html === null || $html === '')) {
+            $email->text('');
+        }
+
+        foreach ($message->getAttachments() as $attachment) {
+            $part = $this->attachmentPart($attachment);
+            if ($part !== null) {
+                $email->addPart($part);
+            }
+        }
+
+        $this->applyThreadHeaders($email, $message);
+        $this->applyCustomHeaders($email, $message);
+
+        return $email;
+    }
+
+    private function attachmentPart(MessagePartInterface $attachment): ?DataPart
+    {
+        $content = $attachment->getContent();
+        if ($content === null) {
+            return null;
+        }
+
+        $part = new DataPart(
+            $content,
+            $attachment->getName(),
+            $attachment->getType() ?? 'application/octet-stream',
+        );
+
+        if ($attachment->getDisposition() === 'inline') {
+            $part->asInline();
+            $cid = $attachment->getContentId();
+            if ($cid !== null && $cid !== '') {
+                $part->setContentId($this->normalizeContentId(trim($cid, '<>')));
+            }
+        }
+
+        return $part;
+    }
+
+    /**
+     * Build a map of original => normalised Content-IDs for inline parts whose
+     * id required normalisation (i.e. lacked an "@").
+     *
+     * @return array<string,string>
+     */
+    private function inlineCidRewrites(MessagePropertiesBaseInterface $message): array
+    {
+        $rewrites = [];
+
+        foreach ($message->getAttachments() as $attachment) {
+            if ($attachment->getDisposition() !== 'inline') {
+                continue;
+            }
+            $cid = $attachment->getContentId();
+            if ($cid === null || $cid === '') {
+                continue;
+            }
+            $original = trim($cid, '<>');
+            $normalized = $this->normalizeContentId($original);
+            if ($original !== $normalized) {
+                $rewrites[$original] = $normalized;
+            }
+        }
+
+        return $rewrites;
+    }
+
+    /**
+     * @param array<string,string> $rewrites
+     */
+    private function rewriteCids(string $html, array $rewrites): string
+    {
+        foreach ($rewrites as $original => $normalized) {
+            $html = str_replace('cid:' . $original, 'cid:' . $normalized, $html);
+        }
+
+        return $html;
+    }
+
+    private function normalizeContentId(string $cid): string
+    {
+        return str_contains($cid, '@') ? $cid : $cid . '@ktrix';
+    }
+
+    private function applyThreadHeaders(Email $email, MessagePropertiesBaseInterface $message): void
+    {
+        $headers = $email->getHeaders();
+
+        $inReplyTo = $message->getInReplyTo();
+        if ($inReplyTo !== null && $inReplyTo !== '') {
+            $headers->addIdHeader('In-Reply-To', $this->stripBrackets($inReplyTo));
+        }
+
+        $references = array_values(array_filter(array_map(
+            fn (string $id): string => $this->stripBrackets($id),
+            $message->getReferences(),
+        ), static fn (string $id): bool => $id !== ''));
+
+        if ($references !== []) {
+            $headers->addIdHeader('References', $references);
+        }
+    }
+
+    private function applyCustomHeaders(Email $email, MessagePropertiesBaseInterface $message): void
+    {
+        $headers = $email->getHeaders();
+
+        foreach ($message->getHeaders() as $name => $value) {
+            if (!is_string($name) || $value === null) {
+                continue;
+            }
+            if (in_array(strtolower($name), self::RESERVED_HEADERS, true)) {
+                continue;
+            }
+            $headers->addTextHeader($name, is_array($value) ? implode(', ', $value) : (string) $value);
+        }
+    }
+
+    /**
+     * @param array<int,AddressInterface> $addresses
+     * @return list<MimeAddress>
+     */
+    private function addresses(array $addresses): array
+    {
+        $result = [];
+        foreach ($addresses as $address) {
+            if ($address instanceof AddressInterface && trim($address->getAddress()) !== '') {
+                $result[] = $this->address($address);
+            }
+        }
+        return $result;
+    }
+
+    private function address(AddressInterface $address): MimeAddress
+    {
+        return new MimeAddress($address->getAddress(), $address->getLabel() ?? '');
+    }
+
+    private function stripBrackets(string $id): string
+    {
+        return trim($id, " \t<>");
+    }
+}