Horde mail users not authorized (Linux)

Running

'tail /usr/local/psa/var/log/maillog'

shows LOGIN FAILED and "unable to open mail authentication database: No such file or directory"
You can verify this by running

'ls /var/lib/plesk/mail/auth/passwd.db'

which will return "No such file or directory".
Fix this by running the following command as root:

/usr/local/psa/admin/sbin/mail_auth_dump