E-mail client
An e-mail client is a frontend computer program used to manage e-mail. Large all-in-one e-mail clients such as Mozilla Thunderbird and Microsoft Outlook today combine the operations of an MTA, MDA, MRA and MUA in one application. However, simple MUAs like mutt are also sometimes referred to as e-mail clients.
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Function of an MUA
A mail user agent (MUA) functions by connecting to a mailbox into which e-mail has been fetched and stored in a particular format. It typically presents a simple user interface to perform tasks with the mail. An MUA by itself is incapable of sending or retrieving mail.
Mail concept
In a POP3 mail setup, an MRA retrieves mail from a remote mailserver. An MDA delivers that mail to a local mailbox. Finally, an MUA connects to the local mailbox and reads mail. An MTA is then called to connect to a remote MTA to send mail.<ref>http://wiki.mutt.org/?MailConcept</ref> However, note that some/ several of these components may be built into the same application. For example, in many MUAs, atleast a basic MTA is built in In an IMAP mail setup, an MDA is unnecessary as the mail stays on the mailserver and is directly read from there.Template:Fact
Standards
While popular protocols for retrieving mail include POP3 and IMAP4, sending mail is usually done using the SMTP protocol.
Another 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.
Most e-mail clients use an X-Mailer header to identify the software used to send the message. According to RFC 2076<ref>http://www.faqs.org/rfcs/rfc2076.html</ref>, this is a common but non-standard header. A Thunderbird extension called dispMUA supports over 500 of these headers and recognizes almost 2000 others. <ref>http://cweiske.de/misc_extensions_dispmuas.htm</ref>
Webmail
In addition to the fat client e-mail clients and small MUAs in cooperation with a local MDA/ MTA/ MRA, there are also Web-based e-mail programs called webmail. Webmail has several advantages which include the ability to send and receive e-mail from anywhere using a single application: a web browser. This eliminates the need to setup an MTA/ MRA/ MDA/ MUA chain. Significant examples of e-mail services which also provide the user a webmail interface are hotmail and gmail.
