Retrieve mailbox password from a Plesk Onyx backup file

If you find yourself working with a new client who only has a Plesk backup file to work with and wants you to create mailboxes on a new server keeping their old usernames and passwords, it can be a bit of a hassle to find the usernames and passwords for the mailboxes they have used. Luckily all this information can be found in the expanded backup itself in a file with this format backup_info_*.xml