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We setup a relay server for Office 365 on a Windows Server 2012 R2 instance roughly a year ago. Everything has been fine until the past week when the relay appears to lock up and e-mails continue to backup in the Queue folder. Restarting the server doesn't resolve the issue and when everything starts working again, I'm unsure as what we've done to fix it. In the Event Log under System there are numerous entries with the following text:

Message delivery to the host '...' failed while delivering to the remote domain '...' for the following reason: The connection was dropped by the remote host.

When we contact Microsoft, they state the issue is not on their end but is due to something with our server. I've came across a few articles/posts stating known issues with Windows Server 2003 and IIS6, but I couldn't find anything regarding a similar issue with Windows Server 2012 R2.

I've attempted to find some more information and/or logs that may have more information regarding the error, but the log(s) for the relay in IIS doesn't appear to have any error information related to this.

As for the current set-up, we have two relays created and listening on different ports. The first is listening on the standard port (25) and allows Anonymous Access. The other is listening on two non-standard ports (e.g. 60125, 65318) and allows both Windows Authentication and Anonymous access. The reason for two is that the Relay Restrictions have different IP Address ranges allowed to send to each. The connector on Office 365 is authenticating via a specified IP address. The Outbound Security for each is set to Anonymous Access (TLS Encryption Unchecked). The Outbound Connections are both set to port 25

Let me know if you'd like some more information regarding the configuration of the SMTP relay(s).

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