Windows Server SMTP role - messages to one specific domain goes into the queue folder

Question

Windows Server SMTP role - messages to one specific domain goes into the queue folder

TPauk 1

I have a Windows 2019 server running the SMTP role (IIS Manager). When I send email via telnet, all email gets processed and sent within seconds except to this one specific domain. Those emails get put in the mailroot/queue folder immediately and they stay there anywhere from 15 minutes to 6 hours. 95% are delivered delayed, but a few we have gotten an NDR for (must have exceed the max retry). I just cant seem to figure it out. We put our SMTP server IP address in our SPF record, and we are even using soft fail in the record too. Any other domain the mail gets sent (gmail, yahoo, outlook.com). The next hop from our SMTP servers is to an Office365 environment which has been configured with a connection filter and connector. Again it accepts all other mail, and extended message trace reports show the delay is coming from the 2019 SMTP server. I'm convinced its has to be DNS, but I'm just not sure where to check. Any guidance is appreciated. The mystery question is how does the SMTP server know my recipient domain so quickly that it doesn't even try to send but it gets put in the queue folder. Other domain recipients i dont even notice them hitting the queue folder because it gets processed so quickly.

2 answers

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Answer 1

Bruce (SqlWork.com) 78,161 Volunteer Moderator

When smtp tries to send mail, it looks up the delivery smtp by DNS name, and tries to send. If not available it leaves the mail in the queue.

Note: they may consider your site spam.

TPauk 1 Reputation point

2022-07-19T12:43:46.45+00:00

The SMTP server sends to the same destination server regardless if I'm sending to gmail, yahoo, outlook.com, or our corporate domain. If office365 would blacklist us, none of the other domains would be delivered within a minute while I'm waiting for the other email to be delivered. Just something about this one domain that is problematic. I still think its a DNS issue somewhere internally.

Answer 2

Yurong Dai-MSFT 2,846 Microsoft External Staff

Hi @TPauk ,

Not sure if your Windows event log contains the following warning:

Event 4006, smtpsvc
"Message delivery to the host 'XXX.XX.XXX.XXX' failed while delivering to the remote domain 'hotmail.com' for the following reason: The remote server did not respond to a connection attempt."

Mail will not be sent correctly if the SMTP service start-up type is set to "Automatic" (i.e. start immediately on boot). However, if the SMTP service is set to manual start and then started manually from either services.msc or from IIS manager, e-mail will leave the queue and get delivered to the recipients.

If "Automatic" start-up is used for the service, any attempts to send e-mail will always result in the e-mail getting stuck in the queue and the event 4006 getting written to the event log.

A work-around is to set the start-up type to "Automatic (Delayed Start)". This will, if you am correctly informed, result in the SMTP service getting started 2 minutes after the last "Automatic" service has been started. This is not an optimal solution, but it'll do for the time being.

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Best regards,
Yurong Dai

TPauk 1 Reputation point

2022-07-19T13:03:57.003+00:00

Yes, that's sorta error we see in the Event System log.

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

Changing the startup to automatic delayed per your suggestion did not help the resolve the issue.

This is just for the one specific domain, All other domains are processed and delivered. Again, just for this one domain that the email is delayed so it's not really rejected from the Office365 side as we had Microsoft look into it. Again, email is delayed anywhere from 5 minutes to 6hrs. It feels DNS related and then the email goes into the retries thus it's delayed. At first, I thought we were being throttled or blacklisted on the Office365 side, but Office365 accepts all other email connections from this SMTP server while we wait for the delayed mail to arrive.
Yurong Dai-MSFT 2,846 Reputation points Microsoft External Staff

2022-07-20T10:21:15.187+00:00
Hi @TPauk ,

I think you can try following method:

Make sure firewall doesn't block port 25. You can temporarily disable the firewall. Check if it can receive emails.

Make sure you have correct Exchange server's FQDN in a Smart Host. Go to IIS Manager -> Default SMTP Virtual Server -> Properties -> Delivery -> Advanced -> Smart Host and check it.

You may want to disable" Internet E-mail Auto-protect" in the Symantec Antivirus Client or disable the antivirus email scanning and to exclude the \Inetpub and %systemroot%\system32\inetsrv from any real time virus scanning.

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Windows Server SMTP role - messages to one specific domain goes into the queue folder

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