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I recently came accross a strange issue with Postfix and Virtualmin on every website hosted on my server when someone tried to send a mail using a contact form. The websites stated that the email had been sent correctly, but the email never reached the inbox.
Checking the logs in /var/log/mail.log I saw that the emails were not being sent and that the following error message was being written to the logs:
Jun 21 09:39:32 server p…