SMTP Failing to relay through Office 365 to one domain

Question

My organization has recently move to utilizing Office 365 as a smart host for our internal SMTP relay. We have a Server 2022 vm running the SMTP service that relays email from internal systems through Office 365. This is working well and we are able to relay email to external domains. However, there is one specific domain that appears to be having an issue. We can send to this domain directly from Office 365 but when we try to relay to it the messages appear to be dropped before being picked up by Office 365. None of the messages show up in the Office 365 traces we've done. I have attached a screenshot of one of the Event Log messages.

Office 365 appears to give us little in the way of diagnostic utilities and I am hoping someone here can point me in the right direction so I can get this resolved.

Answer 1

Hello there,

The first thing I would check is local antivirus/firewall software. Disable it and see if the problem goes away.

It could be Anti virus software configuration issue. You may want to disable" Internet E-mail Auto-protect" in the Symantec Antivirus Client or disable the antivirus email scanning and exclude the \Inetpub and %systemroot%\system32\inetsrv from any real-time virus scanning.

SMTP Virtual server had been configured to relay to "only the list below" but no servers were specified - guess the cause of the dropped connection

Hope this resolves your Query !!

--If the reply is helpful, please Upvote and Accept it as an answer–