Instant Email Notifications is not sending for subscribers

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To the best of my knowledge, if the mail log says that the message was sent to a specific email address, and the recipient does not see the message - the issue might be in the spam filter of the receiving mail server.

Personally I've stopped sending email from a self-hosted mail servers some years ago, if you're attempting this - consider instead using a commercial email delivery service. To that end you will need to configure the email sending backend written for Django, corresponding to the used email provider.