Microsoft delivering to A record for webserver not MX record
Over the past month I've had an increasing number of complaints from people I work with about emails being bounced. The problem was intermittent, most email was getting to them, but some wouldn't.
I used to manage all email through my servers, but have slowly been moving it off to Google Apps for domains for each client, which made this quite difficult to debug: no server logs to check through, and every time I asked for a bounce message to be forwarded the message was minimal - no details, just saying "Email could not be delivered"
Initially I thought that PEBKAC as I could see nothing wrong with email configuration, and it seemed unlikely that google were randomly bouncing email or unable to handle high mail loads (though possible, no one was complaining of this elsewhere).
Then it dawned on me, the majority of senders appeared to be having problems from Hotmail/MSN/Live mail... so I jumped into my Hotmail account and started sending some tests. And things became muddier:
It seemed the email was bounced to one domain handled by a single Google Apps account, but not on another handled by the same account. And it turns out that why I wasn't getting a full error message when asked for them from the senders is because Hotmail puts the error message in an attachment - not very handy for forwarding.
Looking at the details of the error message the error was being generated by vchkpw part of Vpopmail, which I know from experience is part of my mail install - Qmail Toaster - and Google don't use. So the email was being rejected by a server other than Google, quite possibly my server.
I checked the MX records for the domains - all correct, all point to googles servers. Very odd. Also odd that this had only started happening recently on domains that had been moved a year ago.
I thought, maybe they are delivering to the old mail server, so I went and checked the logs there. No sign of attempted delivery there. Most odd.
Then I remember, I'd recently set up a mail server on the system that handles the websites for these systems. Primarily for handling mailing lists it doesn't handle user accounts. I checked into the log files, and there are loads of attempts to deliver to email addresses from hotmail/msn/live and the odd one from BT Internet.
So, Microsoft are attempting to deliver email to the Web server rather than the listed MX servers. Well done Microsoft! I don't know _why_ they are doing this, but a quick hack - binding the Qmail SMTP server to a different IP address to the web server - seems to have resolved the problem. Because Hotmail can't find anything running on port 25 of the web server it must decide that it really has to send to the MX server.
Most odd, most annoying, and really bad that I have to work round a problem with an different email sender. Microsoft once again failing to work by the agreed standards of the internet. Yet another reason to not use Microsoft.