E-mail client
An
e-mail client, also called a
mail user agent (MUA), is a
computer program that is used to read and send
e-mail.
Originally, the MUA was intended to be a simple program to read the user's mail messages, which the
mail delivery agent (MDA) in conjunction with the
mail transfer agent (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
mail submission agent, therefore an MUA would not have to provide any transport-related functions.
Since the various
Microsoft Windows versions intended for home use never provided an MTA, most modern MUAs have to support protocols like
POP3 and
Internet Message Access Protocol (IMAP) to communicate 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
Simple Mail Transfer Protocol (SMTP) 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 called
webmail.
An important standard supported by most e-mail clients is
MIME, which is used to send
binary file e-mail attachments. Attachments are files that are not part of the e-mail proper, but are sent with the e-mail.
Messaging Application Programming Interface (MAPI) is a proprietary Microsoft Windows
application programming interface (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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List of e-mail clients*
Comparison of e-mail clients*
Unicode and e-mail
