We have Office 365 with a hybrid deployment. Our on-premise server is relaying email directly out to the web (not via EOP). domain-a.com is an accepted domain on our on-premise server; domain-b.com is not.
The issue we have had was that the on-premise server was a bit low on space on the C: volume and the transport server was rejecting emails with a from address of domain-b.com, but not domain-a.com. The error message received was:
send-mailmessage : Insufficient system storage. The server response was: 4.3.1 Insufficient system resources
At line:1 char:1
+ send-mailmessage -to [email protected] -Subject test -from [email protected]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (System.Net.Mail.SmtpClient:SmtpClient) [Send-MailMessage],SmtpException
+ FullyQualifiedErrorId : SmtpException,Microsoft.PowerShell.Commands.SendMailMessage
Once I had cleared down the log files in IIS, email was successfully relayed for both domains. What could be the explanation for this? I'm not sure where to start.
Backpressure is caused by low systems resources, which you know that you had. It causes SMTP sessions to be rejected or delayed.
Advice: Move all the logs (IIS, transport protocols, etc) off the system volume.