Home / server / Questions / 314509
Accepted
Brandon
Asked: 2011-09-23 10:24:10 +0800 CST

IIS6 SMTP service attempts to send mail to domain root A records instead of MX, why?

  • 772

Getting some random bouncebacks on e-mail processed through an IIS6 SMTP service acting as a smart host (outbound mail gateway).

Configuration example... For domain example.com we there is a Root A record set to 1.1.1.1, a www A record set to 1.1.1.1, and a single MX record set to mail.thirdpartyhostingcompany.com. Nothing else. An e-mail is sent to [email protected]...

Normal behavior would of course be looking up mail.thirdpartyhostingcompany.com and sending mail to that server, but here's the weird part... In the logs we are getting OutboundConnectionResponse log entries listing 1.1.1.1 as the mail server IP address.

This behavior is random, and re-sending the message usually causes it to be delivered to the proper place with no issue.

Does IIS6 SMTP fallback to a domain root A record in a certain scenario? Why would this behavior occur?

domain-name-system email email-server email-bounces

1 Answers

  1. Best Answer

    That sounds like a DNS problem to me. If an MTA is unable to find an MX record for the domain it will fallback to the A record for the domain.

    This might be hard to track down if it's occurring randomly but you can try using the smtpdiag tool when it's occurring to see if it gives you any clues.

    • 2