Unreliable email delivery

I read this forum via email and I got the impression that email delivery is unreliable: for some posts, I did not get an email notification. This problem is not related to the recent server switch: email notifications have been occasionally lost since the switch to this forum.

I assume that this might be caused by an anti spam measure of my mail service provider: to increase the cost for spammers, email delivery is often rejected with a temporary error message ("please try again later"). As this is a temporary error, a conformant mail server should retry the delivery for some time. Maybe, the retrials are very limited for this forum's mail server (I have not yet observed lost emails from other mail servers).

We are using mailgun to send emails from this forum.

Can you give me a specific set of circumstances in which you noticed not receiving an email?

I will provide details for the next notification I recognize not to have received (I do not recognize all missing notifications).

The latest incident where I believed to have missed a notification is https://community.plone.org/t/upgrade-to-5-2-failing-with-iatcttool-has-no-attribute-iro/8909/9. However, after rereading the post, I recognize that I might have been wrong (the information I missed was not prominent and far down the post - I might have gotten the notification and missed it in the message).

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