What type of performance testing are you thinking about? If you are just looking for relay testing (a'la' SMTP gateway) then pop messages may work fine. If you are looking to test delivery on Lotus Notes (for instance) then you may have a different set of issues to worry about.
Which server and which interfaces will generally drive the selection of which performance testing tool will be appropriate for your use. In the sense of performance testing a mail server (other than the specifics of the business process), the planning component is not unlike that for database or web server testing. Just consider the email server to be a specialized type of database server.