Opensourceinfo

The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. Courier now implements basic web-based calendaring and scheduling services integrated in the webmail module.

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. In style it is similar to Smail 3, but its facilities are more general. There is a great deal of flexibility in the way mail can be routed, and there are extensive facilities for checking incoming mail. Exim can be installed in place of sendmail, although the configuration of exim is quite different to that of sendmail.

msmtp is an SMTP client that can be used with Mutt and other MUAs. It forwards messages to an SMTP server which does the delivery. Features include various SMTP AUTH methods, TLS-encrypted connections (including server certificate verification and the ability to send a client certificate), support for multiple accounts, DSN, and IPv6 support.

What is Postfix? It is a mailer that started life as an alternative to the widely-used Sendmail program. Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.

Sendmail is a Mail Transfer Agent, which is the program that moves mail from one machine to another. Sendmail implements a general internetwork mail routing facility, featuring aliasing and forwarding, automatic routing to network gateways, and flexible configuration.