For some reason we cannot send larger email to one of our customers. Small emails are fine, but even a message with a 1MB attachment fails to deliver for several hours - Postfix logs keep filling with status=deferred (conversation with customer***[IP***] timed out while sending message body), but eventually the messages get sent.
The customer is using "Microsoft ESMTP MAIL Service, Version: 6.0.3790.3959" (checked with telnet to port 25), although I'm not sure if that is really the recipients software fault. We can send email without problems to pretty much everywhere else. The customer has confirmed that they do not have such problem with other people sending them mail. Only mail from our server seems to get stuck.
I've tried lowering the MTU value of our outgoing interface to 1000 and even 500. Other places like this or this show that there is a rare problem, but are without solutions or confirmations.
Below is the Postfix log of a typical message sending process to that customer. (Email addresses replaced with ***, customer mail server with customer*** and ip***)
Aug 18 15:41:28 gozilla postfix/cleanup[14751]: F0DB414080: message-id=<***>
Aug 18 15:41:28 gozilla postfix/qmgr[21922]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 15:44:44 gozilla postfix/qmgr[14985]: F0DB414080: skipped, still being delivered
Aug 18 15:44:45 gozilla postfix/qmgr[15132]: F0DB414080: skipped, still being delivered
Aug 18 15:44:45 gozilla postfix/qmgr[15242]: F0DB414080: skipped, still being delivered
Aug 18 15:44:47 gozilla postfix/smtp[14752]: F0DB414080: to=<***>, relay=customer***[IP***], delay=199, delays=0.64/0/0.68/198, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
Aug 18 15:46:44 gozilla postfix/qmgr[15242]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 15:50:01 gozilla postfix/smtp[15555]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=514, delays=317/0.01/0.17/196, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
Aug 18 15:52:52 gozilla postfix/qmgr[15242]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 15:56:35 gozilla postfix/smtp[15872]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=908, delays=685/0.01/0.16/222, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
Aug 18 16:14:45 gozilla postfix/qmgr[15242]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 16:18:18 gozilla postfix/smtp[17065]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=2210, delays=1997/0/0.17/213, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
Aug 18 16:23:22 gozilla postfix/qmgr[17635]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 16:27:23 gozilla postfix/smtp[17646]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=2755, delays=2515/0.01/0.21/240, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
--- cut out more lines like this ---
Aug 18 19:56:17 gozilla postfix/qmgr[18145]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 19:59:31 gozilla postfix/smtp[26730]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=15484, delays=15289/0.01/0.2/194, dsn=4.4.2, status=deferred (conversation with customer***[IP***] timed out while sending message body)
Aug 18 22:21:17 gozilla postfix/qmgr[18145]: F0DB414080: from=<***>, size=1671905, nrcpt=1 (queue active)
Aug 18 22:26:01 gozilla postfix/smtp[32493]: F0DB414080: to=<***>, relay=customer***[IP***]:25, delay=24274, delays=23989/0.01/0.37/284, dsn=2.6.0, status=sent (250 2.6.0 <***> Queued mail for delivery)
Aug 18 22:26:01 gozilla postfix/qmgr[18145]: F0DB414080: removed