Outlook.com IMAP looses attachment contents on COPY

Outlook.com IMAP looses attachment contents on COPY operation.

Coping an email with an attachment on Outlook.com using IMAP works correct for small attachments:

C: 7554e85569d0465d UID COPY 100828 "Destination"
S: 7554e85569d0465d OK [COPYUID 43582022 100828 100003] COPY completed

With large attachments (~3MB), although COPY command succeeds:

C: 7734b17263a84259 UID COPY 100823 "Destination"
S: 7734b17263a84259 OK [COPYUID 43582022 100823 100001] COPY completed

All contents of the attachments are removed by the server:

------_=_NextPart_001_01CF7412.6651E594
Content-Type: image/jpeg; name="100557_1_DSC_0041 (2).jpg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="100557_1_DSC_0041 (2).jpg"
MS-Blob-Excluded: messageId=7C7BA5A2-E006-11E3-94A8-2C59E5464128; blobId=0; encodedLength=2925184; resolveError=true; blobHash=F206EC6BB4B95BF30B1382C38A33C9BEC3DBFB4A

------_=_NextPart_001_01CF7412.6651E594--

After COPY operation email message is modified by the server:
attachment contents are removed (attachment must be large enough ~3MB)
– new MIME header: MS-Blob-Excluded is added to the MIME entity representing the attachment.

All IMAP clients are affected.

It seems that this behavior is coded-in on purpose, however it’s a bug and violation of the IMAP protocol.

It would be much wiser to fail on COPY command with appropriate error message, if for some reason (performance?) the operation would result in loosing data.

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