I've been scratching my head over this one for the last few hours, so I hope someone can help.
I've got Zimbra running alongside Plesk on the same server. It's been working fine for several months but this morning stopped accepting incoming mail. I have Plesk set up to listen on port 25 and 2525, with Zimbra's MTA on 25. This config has never been a problem.
I've tried disabling Qmail in Plesk so that only Zimbra is listening on 25 but that's not helped. Telnet'ing to 25 on localhost comes back with:
[root@precision1 xinetd.d]# telnet localhost 25
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
220 precision1.precisionstitching.co.uk ESMTP Postfix
The only big thing that's changed is that I updated the rDNS on two IP addresses last night. All standard DNS records are unchanged. Would that stop it from working?
I hope I've provided enough info but please let me know if I've missed anything!