Email Delivery Delays

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upnort
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Email Delivery Delays

Post by upnort » 2020/11/24 17:15:17

CentOS 7, Postfix, Dovecot. Fully updated.

For some days ago I notice some emails from our email server are not being received for several hours. The effect seems random. Some emails punch through immediately, other emails take several hours to be received.

I don't see anything obvious in the logs but this could be me not seeing the tip of my nose. Looking for suggestions and ideas.

We use an external spam filter service. I don't know if that is playing a role.

There was the recent mass CentOS 7.9 patches. I updated the server Nov. 17. There was an updated dovecot package. I don't know if that plays a role. I did not record when the delays started and while coincidence does not mean causation, the latest updates might be related.

Thank you!

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KernelOops
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Re: Email Delivery Delays

Post by KernelOops » 2020/11/24 17:40:37

You did not make it clear where the delay is.

Do you see a delay in dovecot storing or reading emails? (pop/imap/lmtp/etc). Or are you seeing delays in postfix RECEIVING emails? two entirely different services.
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upnort
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Re: Email Delivery Delays

Post by upnort » 2020/11/24 18:16:53

Thanks for the reply. I haven't yet studied the server logs. I see the delay in my (Thunderbird) email client. I am receiving some mails several hours after the mail time stamp. For example, I will receive an email time stamped at 5:45 PM but don't receive the mail until about 10 PM.

Thunderbird is not the problem because there have been no version updates on the system I use Thunderbird.

I plan to send test mails from different servers because I don't know why I receive some mails immediately and others hours later.

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KernelOops
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Re: Email Delivery Delays

Post by KernelOops » 2020/11/24 18:38:42

I'd disable the external spam protection for a while, to test if that is the problem.
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upnort
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Re: Email Delivery Delays

Post by upnort » 2020/11/25 19:23:48

Preliminary tests indicate the delay might be close to or exactly 4 hours. I still have not found a pattern for which servers/subnets are delayed.

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KernelOops
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Re: Email Delivery Delays

Post by KernelOops » 2020/11/26 01:46:19

Check your MX records, maybe the emails are being delivered to the wrong place and the remote servers keep trying until final delivery is successful.
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upnort
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Re: Email Delivery Delays

Post by upnort » 2020/11/26 13:57:37

I'll check, but why would the records change suddenly?

After more digging I discovered that a couple of hours after I performed the big CentOS 7.9 update Nov. 17, the affected servers were seeing delivery delays of 15 minutes. Then on Nov. 20 the delays were 4 hours.

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KernelOops
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Re: Email Delivery Delays

Post by KernelOops » 2020/11/26 15:42:54

It happens, when you have multiple MX's and one of them does dead and if it has a higher priority that others, then you get huge delivery delays.

Another thing to check, is DNS resolution problems. If the remote can't resolve your MX for some reason, you'll get similar delays.
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upnort
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Re: Email Delivery Delays

Post by upnort » 2020/12/01 16:23:47

Well, this is stupid. Another employee modified the external spam filtering service and never told anyone. Sure is fun working in a place with no belief in configuration management.

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