That's correct Cory. Plesk has 3 levels of access. Client (you), Domain User (your clients) and Mail Users.
In the Mailbox section of a mailname, make sure that "Control Panel Access" is cfecked (or something like that) and then the person can login to Plesk with
user@domain.com as username and the mailbox password. They can then manage password, autoresponders, spamassassin and other features.