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. Error in /var/log/maillog:

  NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 454 4.7.1 <mail@example.com>: 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.

• Error in Horde webmail interface when trying to send an email:

  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. Enable mail relay at Tools & Settings > Mail Server Settings > Relay options:
   

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

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