Actually, mail would be by backing up the mailenable directory. As for Plesk, know. Since it integrates with IIS. This means you would have to do a manual backup to file of the configuration in IIS then back it up and back up the Plesk Dir. I believe, but you would have to check with support, that if you restored a system, restored IIS from a backup configuration file from the exact same time plesk was copied, and then copied back over the Plesk dir, that everything would come back up. I'm not sure about this though, so you would want to ask support.
I do a full server backup almost everyday so I don't worry about just backing up plesk. It's quicker to get my systems back up this way.