Some users report, that they don’t get some emails from our discourse instance. We are using “mailing list mode”. I tried to analyze one specific occurrence: approximately four out of 117 users of a group reported that they did not get a given mail. The other people did get the email. The affected users did get other emails from the same category (before and after the affected mail). Looking at the admin panel > email protocol, I can filter for the email address of an affected user and do not see an entry for the given post/email, either in the tab sent, nor in skipped or bounced.
How can I debug further what the root cause might be?
Maybe related (open/no cause found): Weird behaviour for sent emails