Understanding Mail Protocols
Mountain Lion Server uses three standard protocols to send and receive mail:
Users send e-mail with the Simple Mail Transfer Protocol (SMTP).
Users receive e-mail with either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).
You can choose to use IMAP, POP, or both. Different client computers may use one or the other to communicate with the server. You can specify which to use when you create an account on the client (including Mac, iOS, Windows, and Linux). The differences between IMAP and POP follow:
Use POP for a slower server Mac with limited storage. POP downloads e-mail to the client computer and deletes the e-mail from the mail server. The client disconnects from the server as soon as the last message is downloaded. Because clients are connected to the server for only short periods, there is little use of the server’s processor and RAM by a client, and large numbers of users can access the server with minimal impact. And because e-mails are deleted from the server, POP has the advantage of using less server storage space than IMAP. All this makes POP an attractive alternative for a slower server Mac with limited RAM ...