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If a message gets logged in the Warden message log but it does not show up in the users mailbox it's likely that the email was discarded by one of Plesk's mail handlers.

To search the log for discarded messages:

# zgrep discarded /var/log/maillog*
/var/log/maillog.processed:May 17 16:15:18 web8 postfix-local[913896]: message discarded by a mail handler
/var/log/maillog.processed.2.gz:May 11 04:33:58 web8 postfix-local[2121107]: message discarded by a mail handler
/var/log/maillog.processed.3.gz:May  9 07:57:44 web8 postfix-local[3185723]: message discarded by a mail handler
/var/log/maillog.processed.3.gz:May  9 07:57:47 web8 postfix-local[3185815]: message discarded by a mail handler
/var/log/maillog.processed.3.gz:May  9 07:57:50 web8 postfix-local[3185826]: message discarded by a mail handler
/var/log/maillog.processed.3.gz:May  9 07:57:53 web8 postfix-local[3185840]: message discarded by a mail handler

If you see message discarded by a mail handler entries that means that the message failed one of Plesk's mail handler tests and was discarded. Usually this is caused by "Enable DMARC to check incoming mail" or "SPF checking mode" set to "reject email when SPF resolves to fail (deny)" under Tools & Settings -> Mail Server Settings.