The Courier mail server is a mail transfer agent (MTA) server that provides ESMTP, IMAP, POP3, SMAP, webmail, and mailing list services with individual components. It is best known for its IMAP server component.Courier can function as an intermediate mail relay, between an internal LAN and the Internet, or perform final delivery to mailboxes. Courier uses maildirs as its native storage format and can also deliver mail to legacy mailbox files.