An email client (or mail user agent [MUA]) is a computer program that is used to read and send email. Protocols supported by email clients include POP3 and IMAP. IMAP and the updated IMAP4 are optimized for storage of email on the server, while the POP3 protocol generally assumes that the email is downloaded to the client. The SMTP protocol is used by most email clients to send email. In addition to the fat client email clients presented here, there are also web-based email programs such as SquirrelMail, Gmail and Hotmail. America Online also has an email client as a component of its Internet access program.

Another important protocol supported by most email clients is MIME which is used to send binary attachments. Attachments are files that are not part of the email proper, but are sent with the email.

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

Javamail, part of the platform-independent Java API, can be used to develop email clients or servers using any of the standard email protocols.

See also

• List of email clients
• Comparison of email clients