We recently moved offices, and thus changed our public IPs for our Postfix mail server, and I'm now experiencing some very strange behaviour.
The server will work fine for 24 hours or so, but then it will stop accepting outbound email via IMAP, or it will send the mail but the mail will stay in the outbox, and then duplicates will be sent.
Restarting the DNS server running on the same box, then restarting postfix, will fix the problem, and in fact some email (both internal and external) will be delivered to local mailboxes. So it seems like some sort of DNS issue is blocking services somehow. I have DNS lookups turned off in main.cf so I don't know why DNS would be an issue.
I can't find any DNS issues with the server - only the external IP changed and that's NATed on the firewall, so from the server's perspective nothing has changed. The current config is unchanged from before the move, and it worked perfectly for over 2 years before that.
As an additional aside, sshing into the box takes 30+ seconds the first time, as does sudoing, but after that initial attempt it instantly logs in. So, all the symptoms point to a DNS issue but I cannot find anything wrong.
