Mail Client :
An email client (or mail user agent [MUA]) is a computer program that is used to read and send e-mail. Protocols supported by email clients include POP3 and IMAP. 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 email clients to send e-mail.
From :en.wikipedia.org/wiki/Mail_client