Unable to send an email using webmail for domain in Plesk: User unknown in virtual alias table

Symptoms

While trying to send an email from domain example.com using webmail, one of following errors are shown:

CONFIG_TEXT: User unknown in virtual alias table

CONFIG_TEXT: Could not send message to SMTP server. Check you have access to send messages via the server and that all To/CC/BCC addresses are valid

CONFIG_TEXT: Error: SMTP Server rejected email. Returned:550 5.1.1 admin@example.com: Recipient address rejected: User unknown in virtual mailbox table

Cause

Local Postfix database file /var/spool/postfix/plesk/virtual.db contains records about domains with mail hosting not from the Plesk server:

# postmap -s /var/spool/postfix/plesk/virtual
admin@example.com admin@localhost.localdomain

Resolution

1. Connect to the Plesk server via SSH.

2. Recreate/var/spool/postfix/plesk/virtual.db database by executing this command:

   # plesk repair mail