Unable to send emails from a particular domain in Plesk: SMTP Error (554): Failed to add recipient: Sender address rejected: Access denied)

Symptoms

• The following error is displayed when trying to send an email from Roundcube:

  SMTP Error (554): Failed to add recipient "john.doe@example1.com" (5.7.1 <bob.doe@example2.com>: Sender address rejected: Access denied).

• The following error is recorded in /var/log/maillog:

  NOQUEUE: reject: RCPT from localhost[::1]: 554 5.7.1 <bob.doe@example2.com>: Sender address rejected: Access denied; from=<bob.doe@example2.com> to=<john.doe@example1.com> proto=ESMTP helo= 

Cause

The sender domain is blacklisted in Tools & Settings > Mail Server Settings > Black List:

Resolution

1. Log into Plesk;
2. Remove the affected domain example2.com from Tools & Settings > Mail Server Settings > Black List.