We have an old 2008R2 Server with IIS running a SMTP process that is set to relay internal emails to an exchange server (external go out). We have 12 domains setup and all work well apart from one domain. It's configured exactly the same as all the other domains.
With this one specific domain the emails appear to disapear - no logs or indeed email delivered.
I can send emails form the old SMTP server to the domain in question via the the exchange server using telnet.
However Using PS commands like Send-MailMessage to the old mail server do not result in an email being delivered (or any mention in the IIS SMTP logs).
Have tried monitoring with Wireshark - can see logs for all email excect the the one domain that "vanishes"
Has anyone got any ideas on how I can debug this apparant black hole for this one particular domain?