Mail server type

Choose here the type of the external mail server: POP is a widely used standard, supported by the vast majority of ISPs, APOP is a variation that allows login without sending the password in clear over Internet (it's also sometimes called Secure POP) and IMAP is new and more sophisticated, PostArmor supports it but not completely.
An example of IMAP server is mac.com, but it can be used as POP too (not APOP, however).
If you don't know what to choose, just leave the default, as it will work in most cases.