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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
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john
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  • This article may help explain...? [http://technet.microsoft.com/en-us/library/bb201658(v=exchg.150).aspx](http://technet.microsoft.com/en-us/library/bb201658(v=exchg.150).aspx) – Rich701 Oct 15 '14 at 17:17

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
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    I understand what the solution was in general, but it doesn't explain why it works for one and not the other. – john May 21 '14 at 11:04
  • Perhaps they came in on different connectors that have different rules? I don't know, you need to tell us more about your environment. – mfinni Oct 15 '14 at 17:31