Mail Subsystem - Computer Definition
A software package responsible for receiving, delivering, and forwarding email. The mail transport protocol used throughout the Internet is the Simple Mail Transfer Protocol (SMTP). Implementations of this protocol are available from different vendors and public-domain sources. The oldest and still most popular is sendmail. Mail access from client programs such as Outlook, Outlook Express, Eudora, and others can be handled through IMAP and POP3.