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I have a problem with postfix. Our server not accepting the emails from a domains.

On the receiver server logs I see:

 postfix/smtpd[4533]: improper command pipelining after DATA from mail.domain.hu[IP address]:
postfix/smtpd[4533]: lost connection after DATA (0 bytes) from mail.domain.hu[IP address]

On the sending server logs (it uses exim4):

 R=lookuphost T=remote_smtp defer (110): Connection timed out: SMTP timeout while connected to domain [IP]

Does anyone have a suggestion what can cause a problem like this? Can a bad (self-signed) SSL cert on the receiving server cause this?

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    Can you telnet to port 110 on the receiving server from the sending server? – Simon Greenwood Dec 13 '17 at 16:55
  • Could you elaborate on the fail2ban tag on your question? Post your iptables -nL? – anx Dec 13 '17 at 17:39
  • Hi, port 110 is POP3, I think it is not related, the problem is between 2 MTA a postfix and an exim. – Arany Péter Dec 13 '17 at 17:40
  • The fail2ban came because it is very weird: sometimes a message goes though and get delivered. From 20 times 1 time it is working. I though it is a fail2ban problem, but I put the IP of the sending server to whitelist, the problem still exists. And because of I see all the mail delivery attempts in the log I think it is not a firewall / networking problem – Arany Péter Dec 13 '17 at 17:44
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    Are you only allowing TLS smtp connections? Port 25 should except standard, unencrypted communications. Not all servers support or use TLS. – Appleoddity Dec 13 '17 at 19:14

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