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I am really baffled by sendmail sending emails to one particular host. It seems absolutely fine while communicating with other servers, but on a particular host it just hangs at the very end:

050 250 local recipient ok
050 >>> DATA
050 354 Command DATA Start mail input; end with <CRLF>.<CRLF>
050 >>> .

The last line is just hanging for 3-4 minutes before continues to

050 250 message sent ok 
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I'm reminded of this: – Tom O'Connor Feb 12 '13 at 17:32
:) However, the 500 mile mail is about the timeout to connect to the remote server. This is about disconnecting after submitting email. – adamo Feb 13 '13 at 7:20

This look like a DNS timeout issue. I think that the remote server is checking your reverse DNS for the primary IP of your mail server.

It could be an anti-spam policy - to delay incoming connection so that the server is not desirable for spammers.

It could be their DNSBL or other anti-spam or anti-virus modules on their server that are slow.

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It cannot be a DNS timeout because the delay does not happen when the server initiates the connection (and hence it could be a FEATURE(greet_pause) setting or DNS). This happens after the sender issues the final dot, signaling the end of message. It may be due to content filtering in combination with heavy load on this server. – adamo Feb 13 '13 at 7:17
@adamo, I am thinking that it could be possible that it is doing a reverse DNS query for the mail header, or envelope. – Mircea Vutcovici Feb 13 '13 at 16:03
I agree to that. – adamo Feb 13 '13 at 22:14

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