SharePoint 2016 Outbound SMTP Failures

Recently I was configuring a SharePoint 2016 farm, and encountered some peculiar issues with outbound email.

SharePoint 2016 is the first version of SharePoint to include built-in support for TLS. In any previous version of SharePoint, TLS requirements were fulfilled by setting up a SMTP relay capable of authenticating to the desired target SMTP server.

Interestingly, It seems that SharePoint 2016 also responds to SMTP authentication challenges despite not having an explicit configuration option in Central Administration for which credentials to use for SMTP.

The issue I recently experienced is as follows:

  • List / Library “initial” alert subscription messages are delivered to the appropriate address
  • Actual alerts from a list / library are not delivered
  • Workflow Task emails are not delivered

Digging into the ULS logs of the SharePoint server, I noticed the following:

  • Messages send by w3wp (running under the web app pool service account) were delivered
  • Messages sent by OWSTIMER (running under the farm account) were not delivered.  The timer job in question is “job-immediate-alerts.”

So, despite having outbound email configured in Central Administration, it seems that SharePoint is not treating different classes of outbound email equally.

I tried many of the “well known fixes” to no avail:

  • Re-starting the server
  • Re-starting the timer service
  • Manually starting the job-immediate-alerts timer job with PowerShell
  • Altering the alerts properties of the site with stsadm

I finally broke out WireShark on my SharePoint server to observe the SMTP traffic.  What I found was interesting:

  • Messages sent by w3wp.exe had these characteristics:
    • SharePoint sends the message immediately upon request from the browser to subscribe to alerts on a library
    • SharePoint opens a SMTP session to the configured server
    • The Exchange 2013 server responds with an SMTP ntlm authentication challenge
    • The SharePoint server provides the credentials of the web app service account!
    • Exchange returns with smtp 5.7.1 client was not authenticated. 
    • SharePoint ignores the 5.7.1 error message, and delivers the message anyway
  • Message sent by OWSTIMER.exe had these characteristics:
    • SharePoint attempts to send the message with each execution of the job-immediate-alerts timer job.
    • SharePoint opens a SMTP session to the configured server
    • The Exchange 2013 server responds with an SMTP ntlm authentication challenge
    • The SharePoint server provides the credentials of the farm service account!
    • Exchange returns with smtp 5.7.1 client was not authenticated. 
    • SharePoint stops attempting to deliver the message because of the error!

In both of these scenarios, neither the farm service account, nor the web app service account are configured with Exchange mailboxes, so the authentication fails.

The receive connector in Exchange is configured to allow TLS, Exchange Authentication, and Anonymous authentication.

The unexpected behavior is this: SharePoint reacts to an SMTP 5.7.1. unauthenticated message differently depending on the context from which the SMTP session was initiated.  SMTP sessions initiated directly in the web app context succeed, but SMTP sessions initiated from timer jobs fail.

My temporary solution was to create a separate receive connector in Exchange on a separate port scoped so to only the SharePoint server’s IP that allows only anonymous authentication (it seems that by having Exchange Authentication checked, SharePoint fails).  This causes the Exchange server to never prompt the SharePoint server for STMP authentication, and therefore messages are delivered.

I’ll update this post as I discover more.

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