diff --git a/src/PhpImap/Mailbox.php b/src/PhpImap/Mailbox.php
index 343e876a..011b497d 100644
--- a/src/PhpImap/Mailbox.php
+++ b/src/PhpImap/Mailbox.php
@@ -1432,6 +1432,14 @@ public function getMail(int $mailId, bool $markAsSeen = true): IncomingMail
         return $mail;
     }
 
+    protected function hasAttachmentDisposition(object $partStructure): bool
+    {
+        /** @var scalar|array|object|resource|null */
+        $disposition = $partStructure->disposition ?? null;
+
+        return \is_string($disposition) && 'attachment' === \mb_strtolower($disposition);
+    }
+
     /**
      * Download attachment.
      *
@@ -1446,7 +1454,7 @@ public function getMail(int $mailId, bool $markAsSeen = true): IncomingMail
      */
     public function downloadAttachment(DataPartInfo $dataInfo, array $params, object $partStructure, bool $emlOrigin = false): IncomingMailAttachment
     {
-        if ('RFC822' == $partStructure->subtype && isset($partStructure->disposition) && 'attachment' == $partStructure->disposition) {
+        if ('RFC822' == $partStructure->subtype && $this->hasAttachmentDisposition($partStructure)) {
             $fileName = \strtolower($partStructure->subtype).'.eml';
         } elseif ('ALTERNATIVE' == $partStructure->subtype) {
             $fileName = \strtolower($partStructure->subtype).'.eml';
@@ -1943,9 +1951,7 @@ protected function initMailPart(IncomingMail $mail, object $partStructure, $part
 
         $isAttachment = isset($params['filename']) || isset($params['name']) || isset($partStructure->id);
 
-        $dispositionAttachment = (isset($partStructure->disposition) &&
-            \is_string($partStructure->disposition) &&
-            'attachment' === \mb_strtolower($partStructure->disposition));
+        $dispositionAttachment = $this->hasAttachmentDisposition($partStructure);
 
         // ignore contentId on body when mail isn't multipart (https://github.com/barbushin/php-imap/issues/71)
         if (
@@ -1961,7 +1967,7 @@ protected function initMailPart(IncomingMail $mail, object $partStructure, $part
         }
 
         // check if the part is a subpart of another attachment part (RFC822)
-        if ('RFC822' === $partStructure->subtype && isset($partStructure->disposition) && 'attachment' === $partStructure->disposition) {
+        if ('RFC822' === $partStructure->subtype && $dispositionAttachment) {
             // Although we are downloading each part separately, we are going to download the EML to a single file
             //incase someone wants to process or parse in another process
             $attachment = self::downloadAttachment($dataInfo, $params, $partStructure, false);
@@ -1993,14 +1999,14 @@ protected function initMailPart(IncomingMail $mail, object $partStructure, $part
 
         if (!empty($partStructure->parts)) {
             foreach ($partStructure->parts as $subPartNum => $subPartStructure) {
-                $not_attachment = (!isset($partStructure->disposition) || 'attachment' !== $partStructure->disposition);
+                $not_attachment = !$dispositionAttachment;
 
                 if (TYPEMESSAGE === $partStructure->type && 'RFC822' === $partStructure->subtype && $not_attachment) {
                     $this->initMailPart($mail, $subPartStructure, $partNum, $markAsSeen);
                 } elseif (TYPEMULTIPART === $partStructure->type && 'ALTERNATIVE' === $partStructure->subtype && $not_attachment) {
                     // https://github.com/barbushin/php-imap/issues/198
                     $this->initMailPart($mail, $subPartStructure, $partNum, $markAsSeen);
-                } elseif ('RFC822' === $partStructure->subtype && isset($partStructure->disposition) && 'attachment' === $partStructure->disposition) {
+                } elseif ('RFC822' === $partStructure->subtype && $dispositionAttachment) {
                     //If it comes from am EML attachment, download each part separately as a file
                     $this->initMailPart($mail, $subPartStructure, $partNum.'.'.($subPartNum + 1), $markAsSeen, true);
                 } else {
diff --git a/tests/unit/Fixtures/Mailbox.php b/tests/unit/Fixtures/Mailbox.php
index ee97980e..df5f3414 100644
--- a/tests/unit/Fixtures/Mailbox.php
+++ b/tests/unit/Fixtures/Mailbox.php
@@ -68,4 +68,9 @@ public function getImapOpenOptionsForTests(): int
     {
         return $this->getImapOpenOptions();
     }
+
+    public function hasAttachmentDispositionForTests(object $partStructure): bool
+    {
+        return $this->hasAttachmentDisposition($partStructure);
+    }
 }
diff --git a/tests/unit/MailboxTest.php b/tests/unit/MailboxTest.php
index 75bf7356..30f39b7f 100644
--- a/tests/unit/MailboxTest.php
+++ b/tests/unit/MailboxTest.php
@@ -420,6 +420,39 @@ public function testSetAttachmentsIgnore(bool $paramValue): void
         $this->assertEquals($mailbox->getAttachmentsIgnore(), $paramValue);
     }
 
+    public function testHasAttachmentDispositionRecognizesMixedCaseAttachment(): void
+    {
+        $partStructure = (object) [
+            'disposition' => 'Attachment',
+        ];
+
+        $this->assertTrue($this->getMailbox()->hasAttachmentDispositionForTests($partStructure));
+    }
+
+    public function testDownloadAttachmentTreatsMixedCaseRfc822DispositionAsEmlAttachment(): void
+    {
+        $mailbox = new Fixtures\Mailbox($this->imapPath, $this->login, $this->password, null, $this->serverEncoding);
+        $dataInfo = new Fixtures\DataPartInfo($mailbox, 1, '2', 0, 0);
+        $dataInfo->setData("From: sender@example.com\r\n\r\nAttachment body");
+
+        $partStructure = (object) [
+            'type' => Mailbox::PART_TYPE_TWO,
+            'subtype' => 'RFC822',
+            'disposition' => 'Attachment',
+            'bytes' => 42,
+            'encoding' => 0,
+            'ifid' => 0,
+            'ifsubtype' => 1,
+            'ifdescription' => 0,
+        ];
+
+        $attachment = $mailbox->downloadAttachment($dataInfo, [], $partStructure);
+
+        $this->assertSame('rfc822.eml', $attachment->name);
+        $this->assertSame('Attachment', $attachment->disposition);
+        $this->assertFalse($attachment->emlOrigin);
+    }
+
     /**
      * Provides test data for testing encoding.
      *