When mail is disabled for a particular domain, options that are mail-only are still shown when you click on "Edit FTP Users", such as "Additional email address" and "Login Permissions" showing "Mail" in the drop down box.
Also, it appears that these FTP users are still able to send and receive (or at least authenticate against the pop server) mail even though "Mail for Domain" is turned off. They authenticate against Postfix/saslauthd as if this option was never disabled and are able to send mail. Is this by design? What does disabling the Mail for Domain feature actually disable?