There are a lot of emails stuck in the mail queue shown in Plesk in Tools & Settings > Mail Server Settings > Mail queue.
All stuck emails were sent to a certain domain example.com.
Both automatic connection by the mail server and the manual connection to the receiving mail server fails:
Automatic connection by the mail server - the following error is found in the file
hostname postfix/qmgr: 0CEFF7E685: removed
hostname postfix/qmgr: 99B1A7E666: [email protected], size=705, nrcpt=1 (queue active)
hostname postfix/qmgr: C9B817E66C: [email protected], size=1219, nrcpt=1 (queue active)
hostname postfix/smtp: connect to mail.example.com[203.0.113.2]:25: Connection timed out
hostname postfix/smtp: 99B1A7E666: [email protected], 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)
# telnet mail.example.com 25
telnet: connect to address 203.0.113.2: Connection timed out
Firewall configuration on the recipient mail server blocks connections from the Plesk server.
MX records list for the recipient domain contains outdated record(s).
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.