Mail forwarding does not work on Plesk server if email address was created without password

Symptoms

• Mailbox is enabled for email account, however password was not set, therefore there is no link to log into WebMail:
  
  
  
• Mailbox john.doe@example.com was created with unchecked Access to the Customer Panel option:
  
• Emails are not delivered to the mailbox john.doe@example.com with the following error in /var/log/maillog:

  CONFIG_TEXT: dovecot_authdb_plesk[5239]: No such user 'john.doe@example.com' in mail authorization database

• Forwarding from john.doe@example.com is not working with the following error in /var/log/maillog:

  CONFIG_TEXT: postfix/pipe[54766]: to=<john.doe@example.com>, relay=plesk_virtual, delay=0.3, delays=0.27/0/0/0.03, dsn=4.2.1, status=deferred (Message can not be delivered at this time )

Cause

For such kind of setup, either password should be set with mailbox enabled or there should be no mailbox enabled.

Ability to create email accounts with the mailbox enabled and without a password with disabled checkbox "Access to the Customer Panel" is Plesk bug PPPM-8076.

It is planned to be fixed in future product updates.
Follow the article to be notified when the bug is fixed - the article will be updated.

Resolution

1. Log into Plesk;
2. Go to Plesk >  Domains > example.com > Email Addresses > john.doe@example.com;
3. Either disable mailbox for this account or specify the password for it:
   

Note: With disabled mailbox only email forwarding will be working on the account.

Additional Information

Email cannot be delivered: No such user in mail authorization database