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The function of an MTA is usually complemented with some means for email clients to access stored messages.

This function typically employs a different protocol.

The most widely implemented open protocols for the MUA are the Post Office Protocol ( POP3 ) and the Internet Message Access Protocol ( IMAP ), but many proprietary systems exist ( Exchange, Lotus Domino / Notes ) for retrieving messages.

Many systems also offer a web interface for reading and sending email that is independent of any particular MUA.