618Part VRunning Servers .SMTP The Simple Mail Transfer Protocol (Top web site)
618Part VRunning Servers .SMTP The Simple Mail Transfer Protocol is a network-based protocol thatallows for transmission of messages between systems. .Local IPC Interprocess communications are often used instead of SMTPwhen transferring a message between programs within a system. Upon receiving a message, the MTA places it in a queue to be processed by an MDA(Mail Delivery Agent). Mail Delivery Agents come in two varieties: .LocalMDAs, which deliver messages to mailboxes on the local server. Simpleversions (such as sendmail s mail.local) copy messages directly to a speci- fied mailbox, while complex implementations (such as procmail and maildrop) can alter messages or delivery parameters based on user-specified rules. .RemoteMDAs, which deliver messages over the network to remote servers. Full remote MDAs use DNS (the Domain Name System) to determine the mailexchanger hosts for recipient addresses and deliver to the best one availablefor each. Simple remote MDAs (sometimes also called null clients ) forwardmessages to a central server to continue the delivery process. Most remoteMDAs are capable of either method and will act as configured by the adminis- trator that performed the configuration. You will often see the term MTA used in reference to the software that performsboth MTA and MDA functions. This is a carry-over from older designs that did notseparate the functions and is still fairly accurate given the fact that most mail serverimplementations include a minimum of an MTA, remote MDA, and basic local MDA. When a message reaches its destination server, it is written to the user s mailbox bythe local MDA. From that point, the message may be viewed by the user using oneof three methods: .Direct access to the mailbox An MUA (Mail User Agent) with access to themailbox file, directly or through a network file system, can read messagesfrom the disk and display them for the user. This is generally a console orwebmail application running on the server. .Downloaded to a workstation for local viewing Most mail users use POP3(Post Office Protocol, version 3) to download messages to their local computersand view them in applications such as Evolution or Balsa. By default, messagesare removed from the server during this process (similar to when you get your snail mail from the mailbox). .Accessed interactively over the network Most clients also support viewingmessages while they are still on the server, through IMAP (Internet MessageAccess Protocol). Unlike POP3, this protocol enables users to access multiplefolders on their servers and also allows them to access their messages fromanywhere. However, this also creates a heavier burden on the server becauseit must process (and store) everything that the user decides to keep. Mostwebmail applications use IMAP as their backend protocol for accessing mail- boxes; this eliminates the need for direct access to the mail files and makes iteasier to split functions between systems. Note33_
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