The IMAP user’s computer can ask the server for message headers, ask for the bodies of specified messages, or search for messages that meet certain criteria. These messages are downloaded as the user opens them.
IMAP connections are persistent and remain open, maintaining load on the server and possibly the network as well.
Post Office Protocol (POP)
POP is used only for receiving mail, not for sending mail.
The POP service is like a post office, storing mail and delivering it to a specific address. Mail service stores incoming POP mail until users connect to Mail service and download their waiting mail.
After a user’s computer downloads POP mail, the mail is stored only on the user’s computer. The user’s computer disconnects from Mail service, and the user can read, organize, and reply to the received POP mail.
An advantage of using POP is that your server doesn’t need to store mail that users have downloaded. Therefore, your server doesn’t need as much storage space as it would using IMAP.
However, because the mail is removed from the server, if the user’s computer sustains hard disk damage and loses mail files, there’s no way to recover these files without using data backups.
Another advantage of POP is that POP connections are transitory. After mail is transferred, the connection is dropped and the load on the network and mail server is removed.
POP isn’t the best choice for users who access mail from more than one computer, such as a home computer, an office computer, and a laptop while on the road. When a user retrieves mail via POP, the mail is downloaded to the user’s computer and is usually removed from the server. If the user logs in later from a different computer, the user can’t see previously downloaded mail.
User Interaction with Mail Service
Mail is delivered to its final recipient using a mail user agent (MUA). MUAs are usually referred to as mail clients or mail applications. These mail clients often run on the user’s local computer.
Each user’s mail application must be configured to send messages to the outgoing server and receive messages from the incoming server. These configurations can affect your server’s processing load and available storage space. For more information, see “Configuring Mail Client Applications” on page 60.
Users can also access mail through Webmail. For more information, see “Mail Service Filtering” on page 35.
Chapter 1 Understanding Mail Service
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