if you want to delete mails from plesk older than 30 days here the command
# find /var/qmail/mailnames/*/*/Maildir/*/ -name \* -type f -mtime +30 -exec rm {} \;
Attention in classroom
if you want to delete mails from plesk older than 30 days here the command
# find /var/qmail/mailnames/*/*/Maildir/*/ -name \* -type f -mtime +30 -exec rm {} \;