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 184.108.40.206, a
www A record set to 220.127.116.11, and a single MX record set to mail.thirdpartyhostingcompany.com. Nothing else. An e-mail is sent to
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 18.104.22.168 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?