Forwarded emails not forwarding (Plesk)

I came across an issue with a customer not being able to forward emails for root@hostname and postmaster@hostname, even after setting it up in Plesk and also trying it manually via the command line.

One of the errors showing in qmails error logs was the following:

Uh-oh:_first_line_of_.qmail_file_is_blank.

I found that the following files were blank:

/var/qmail/alias/.qmail-root
/var/qmail/alias/.qmail-postmaster

After adding an email address to these, emails forwarded without issue.

# cat /var/qmail/alias/.qmail-root
email-to-forward-to@domain.com