618 Part V . Running (Web hosts) Servers . SMTP The
618 Part V . Running Servers . SMTP The Simple Mail Transfer Protocol is a network-based protocol that allows for transmission of messages between systems. . Local IPC Interprocess communications are often used instead of SMTP when 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: . Local MDAs, which deliver messages to mailboxes on the local server. Simple versions (such as sendmail s mail.local) copy messages directly to a specified mailbox, while complex implementations (such as procmail and maildrop) can alter messages or delivery parameters based on user-specified rules. . Remote MDAs, which deliver messages over the network to remote servers. Full remote MDAs use DNS (the Domain Name System) to determine the mail exchanger hosts for recipient addresses and deliver to the best one available for each. Simple remote MDAs (sometimes also called null clients ) forward messages to a central server to continue the delivery process. Most remote MDAs are capable of either method and will act as configured by the administrator that performed the configuration. You will often see the term MTA used in reference to the software that performs both MTA and MDA functions. This is a carry-over from older designs that did not separate the functions and is still fairly accurate given the fact that most mail server implementations 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 by the local MDA. From that point, the message may be viewed by the user using one of three methods: . Direct access to the mailbox An MUA (Mail User Agent) with access to the mailbox file, directly or through a network file system, can read messages from the disk and display them for the user. This is generally a console or webmail 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 computers and view them in applications such as Evolution or Balsa. By default, messages are 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 viewing messages while they are still on the server, through IMAP (Internet Message Access Protocol). Unlike POP3, this protocol enables users to access multiple folders on their servers and also allows them to access their messages from anywhere. However, this also creates a heavier burden on the server because it must process (and store) everything that the user decides to keep. Most webmail applications use IMAP as their backend protocol for accessing mailboxes; this eliminates the need for direct access to the mail files and makes it easier to split functions between systems. Note
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