Yepper, I got bonked by sendmail overnight.
I put an IP address in an MX record, and didn’t put a period at the end. You know, like you do in A records. Named-checkzone thought this was fine. But sendmail, in it’s infinite wisdom decided that since there was no period on the damn thing, that it would append the domain name to it. So now I had servers all over the net trying to deliver mail to 126.96.36.199.therichards.org instead of 188.8.131.52.
As we all know sendmail has no bugs, so this can’t be one. Just don’t put an IP address in an MX record.