E-mail client

An e-mail client (or mail user agent ^*) is a computer program that is used to read and send e-mail.

Originally, the MUA was intended to be just a little program to read the messages, which the MDA in conjunction with the MTA would transfer into a local mailbox.

The most important mailbox formats are mbox and Maildir. These rather simple protocols for locally storing e-mails make import, export and backup of mailfolders quite easy.

E-mails to be sent would be handed over to the MTA (perhaps via an MSA), therefore an MUA would not have to provide any transport-related functions.

Since the various Windows versions intended for home use never provided an MTA, most modern MUAs have to support protocols like POP3 and IMAP to comunicate with a remote MTA located at the e-mail providers machine.

IMAP and the updated IMAP4 are optimized for storage of e-mail on the server, while the POP3 protocol generally assumes that the e-mail is downloaded to the client. The SMTP protocol is used by most e-mail clients to send e-mail.

In addition to the fat client e-mail clients (or small MUAs in cooperation with a local MDA/MTA) presented here, there are also Web-based e-mail programs ("webmail").

An important standard supported by most e-mail clients is MIME, which is used to send binary attachments. Attachments are files that are not part of the e-mail proper, but are sent with the e-mail.

MAPI (Messaging Application Programming Interface) is a proprietary Microsoft Windows API which can be used to access the Microsoft Exchange e-mail server or to interact with the Microsoft Outlook client.

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See also