#7 Mail sender close connection before SMTP DATA result returned, mail server drops message.

[Olivier Mascia]

It looks like the SMTP transmission takes some (unusual) shortcuts: it correctly connects, authentify, start sending the message (see DATA command), but then closes the TCP connection as soon as transmission complete, without waiting for the DATA command answer from the server. The server encounters an error (connection reset) when trying to say "thanks I got it (or so)", and as a consequence DROPS the message without delivering it, because this is an unusual behavior, usually only seen from spamming engines which care about nothing except being really quick.

mail server log excerpt below:

Fri 2013-12-27 19:44:11: [5270:1] <-- DATA
Fri 2013-12-27 19:44:11: [5270:1] Creating temp file (SMTP): c:\storage\mdaemon\queues\temp\35\md50000000009.tmp
Fri 2013-12-27 19:44:11: [5270:1] --> 354 Enter mail, end with <CRLF>.<CRLF>
Fri 2013-12-27 19:44:11: [5270:1] Message size: 10513 bytes
Fri 2013-12-27 19:44:11: [5270:1] Passing message through AntiVirus (Size: 10513)...
Fri 2013-12-27 19:44:11: [5270:1] * Message is clean (no viruses found)
Fri 2013-12-27 19:44:11: [5270:1] ---- End AntiVirus results
Fri 2013-12-27 19:44:11: [5270:1] Socket error sending response to DATA
Fri 2013-12-27 19:44:11: [5270:1] * Winsock Error 10054 Connection was reset by the other side!
Fri 2013-12-27 19:44:11: [5270:1] SMTP session terminated (Bytes in/out: 10650/549)