Plesk backup task finishes with the warning: Could not back up all data from the directory '/var/qmail/mailnames/example.com'

Symptoms

Plesk backup task finishes with the warning:

PLESK_INFO: Warning: Mail "example.com"
Could not back up all data from the directory '/var/qmail/mailnames/example.com': 
john_doe: Cannot open: Permission denied
john_doe/.dovecot.sieve: Cannot stat: Permission denied
...

Cause

Directories and files of a mailbox have Incorrect permissions.

Resolution

1. Connect to your Plesk server via SSH.

2. Set popuser as a user and group for the affected mailbox:

   # chown -R popuser:popuser /var/qmail/mailnames/example.com/john_doe