Symptoms
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Emails sent from a Plesk server to particular domains are not delivered and lay in the mail queue as deferred in Plesk at Tools & Settings > Mail Server Settings > Mail Queue.
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One of the following error messages appear in
/var/log/maillog:0572C2801E1: from=john.doe@example.com, size=3826, nrcpt=1 (queue active)
0572C2801E1: host mx.example.net refused to talk to me: 554 IP=IP_ADDRESS - A problem occurred. (Ask your postmaster for help or to contact info@mx.example.net to clarify.) (BL)
connect to example.com[203.0.113.2]:25: Connection refused E37F6A0AF5: to=<notify@example.com>, relay=none, delay=371, delays=371/0.02/0.02/0, dsn=4.4.1, status=deferred (connect to example.com[203.0.113.2]:25: Connection refused)
3B0F8A08BF: host example.com[203.0.113.2] refused to talk to me: 554 5.7.1 You are not allowed to connect.
(host server.example.com[203.0.113.] said: 452 4.7.1 Your IP subnet has been temporarily deferred. Please retry this message later. H:GRM (in reply to end of DATA command))
Cause
Error code 554 means IP-Blocking: A remote mail server is not currently accepting emails from a server IP address. This IP blocking may be due to a previously observed unusual volume of emails at the IP address concerned.
Resolution
Contact the administrator of remote mail server/mail relay server to find out the cause.
For more information about a blacklisted mail server, visit this KB article.