The joys of running one's own mail server

Minor issue.
We changed IP addresses at work recently. A larger block of public facing IPs for more functionality.
And in doing so, we updated almost everything correctly.
Yes, almost everything.
Almost.
Everything.
The one, minor … trivial … thing we left unchanged … broke our support site sending reply emails correctly. They were rejected about 50% of the time, as they came in on the wrong port/ip.
So for the last week we’ve been sending extra emails. And tonight I got pissed off enough that I actually fixed it.
Joy.