[Troubleshooting] Mail users on Plesk server are unable to receive or send messages: 454 4.7.1 Relay access denied

Symptoms

• Unable to send or receive email messages on Plesk server. The following error appears in log file /usr/local/psa/var/log/maillog:

  NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 454 4.7.1 <[email protected]>: Relay access denied;
   /usr/lib64/plesk-9.0/psa-pc-remote[9425]: Message aborted.
   /usr/lib64/plesk-9.0/psa-pc-remote[9425]: Message aborted.

• The following error is shown in Horde webmail interface when trying to send an email:

  PLESK_INFO: There was an error sending your message: Could not open secure TLS connection to the server

Cause

Mail relay is disabled server-wide / mail service is disabled for specific subscription / the subscription is disabled in Plesk UI.

Resolution

1. Log in to Plesk GUI

2. Check if mail relay is enabled at  Plesk > Tools & Settings > Mail Server Settings > Relay options and enable it if it is disabled:
   

3. Make sure that affected subscription is enabled at Plesk > Subscriptions > example.com:
   
   

4. Make sure that mail service is enabled for the affected subscription at Plesk > Subscriptions > example.com > Mail > Change Settings >  Activate mail service on this domain: