Symptoms
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There are a lot of emails stuck in the mail queue shown in Plesk in Tools & Settings > Mail Server Settings > Mail queue.
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All stuck emails were sent to a certain domain example.com.
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Both automatic connection by the mail server and the manual connection to the receiving mail server fails:
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Automatic connection by the mail server - the following error is found in the file
/var/log/maillog:hostname postfix/qmgr[7584]: 0CEFF7E685: removed
hostname postfix/qmgr[7584]: 99B1A7E666: from=jdoe@example.net, size=705, nrcpt=1 (queue active)
hostname postfix/qmgr[7584]: C9B817E66C: from=jdoe@example.net, size=1219, nrcpt=1 (queue active)
hostname postfix/smtp[12133]: connect to mail.example.com[203.0.113.2]:25: Connection timed out
hostname postfix/smtp[12133]: 99B1A7E666: to=john@example.com, relay=none, delay=137241, delays=137121/0.01/120/0, dsn=4.4.1, status=deferred (connect to mail.example.com[203.0.113.2]:25: Connection timed out) -
Manual connection:
# telnet mail.example.com 25
Trying 203.0.113.2...
telnet: connect to address 203.0.113.2: Connection timed out
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Cause
Firewall configuration on the recipient mail server blocks connections from the Plesk server.
MX records list for the recipient domain contains outdated record(s).
Resolution
Contact the owners of the recipient domain example.com so they revise the MX records list to contain correct IP addresses and allow connections over port 25 on the firewalls of their mail servers.