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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 me@domain-a.com -Subject test -from email@domain-b.com                       
    + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                             
        + 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.

BastianW
  • 2,896
john
  • 2,005

1 Answers1

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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.

mfinni
  • 36,892