diff --git a/lib/Providers/Mail/Service.php b/lib/Providers/Mail/Service.php
index 3edea6f..2bbb900 100644
--- a/lib/Providers/Mail/Service.php
+++ b/lib/Providers/Mail/Service.php
@@ -880,7 +880,31 @@ class Service implements ServiceBaseInterface, ServiceMutableInterface, ServiceC
             );
         }
 
-        $nativeMessage = $this->normalizeMessageProperties($message)->toJmap();
+        $nativeProperties = $this->normalizeMessageProperties($message);
+        $attachments = $nativeProperties->getAttachments();
+        foreach ($attachments as $attachment) {
+            if ($attachment->getBlobId() !== null) {
+                continue;
+            }
+
+            $content = $attachment->getContent();
+            if ($content === null) {
+                throw new \InvalidArgumentException(sprintf(
+                    'Attachment "%s" has neither content nor a JMAP blob identifier',
+                    $attachment->getName() ?? 'unnamed',
+                ));
+            }
+
+            $attachment->setBlobId($this->mailService->blobDeposit(
+                $attachment->getType() ?? 'application/octet-stream',
+                $content,
+            ));
+        }
+        if ($attachments !== []) {
+            $nativeProperties->setAttachments(...$attachments);
+        }
+
+        $nativeMessage = $nativeProperties->toJmap();
         $transportId = $this->mailService->entitySubmitFresh($identityId, $nativeMessage, $preSendTarget->collection(), $postSentTarget->collection());
 
         return new EntitySubmitResult(
diff --git a/lib/Service/Remote/RemoteMailService.php b/lib/Service/Remote/RemoteMailService.php
index b9a81a6..7313951 100644
--- a/lib/Service/Remote/RemoteMailService.php
+++ b/lib/Service/Remote/RemoteMailService.php
@@ -929,6 +929,19 @@ class RemoteMailService {
 		$submission->message('#' . $messageCreateId);
 
 		$bundle = $this->dataStore->perform([$r0, $r1]);
+		$messageResponse = $bundle->response(0);
+		if ($messageResponse instanceof ResponseException) {
+			throw new Exception($messageResponse->type() . ': ' . $messageResponse->description(), 1);
+		}
+		if (method_exists($messageResponse, 'createFailure')) {
+			$failure = $messageResponse->createFailure($messageCreateId);
+			if (is_array($failure)) {
+				$type = $failure['type'] ?? 'unknownError';
+				$description = $failure['description'] ?? 'Email creation failed';
+				throw new Exception($type . ': ' . $description, 1);
+			}
+		}
+
 		$response = $bundle->response(1);
 		if ($response instanceof ResponseException) {
 			if ($response->type() === 'unknownMethod') {
@@ -940,6 +953,18 @@ class RemoteMailService {
 		return $this->extractSubmissionIdentifier($response, $submissionCreateId);
 	}
 
+	/**
+	 * Upload attachment content and return its JMAP blob identifier.
+	 */
+	public function blobDeposit(string $type, string $data): string {
+		$response = json_decode($this->dataStore->upload($this->dataAccount, $type, $data), true);
+		if (!is_array($response) || !isset($response['blobId']) || !is_string($response['blobId']) || $response['blobId'] === '') {
+			throw new Exception('JMAP attachment upload did not return a blob identifier', 1);
+		}
+
+		return $response['blobId'];
+	}
+
 	/**
 	 * Submit an existing draft message.
 	 *