Courier is a modular multiprotocol mail server that's designed to
strike a balance between reasonable performance, flexibility and
features:

 * Can be configured to function as an intermediate mail relay, or
   as a mail server that receives mail for one or more domains, or
   anything in between.
 * Web-based administration and configuration tool.
 * Uses an efficient maildir format as its native mail storage
   format. Some support is provided for legacy mbox mailboxes.
 * STARTTLS ESMTP extension (as well as IMAP/POP3/Webmail over SSL)
   in both the client and the server (requires OpenSSL). The ESMTP
   client can optionally require that the remote server's X.509
   certificate is signed by a trusted root CA (a default set of
   root CAs is provided).
 * Mailboxes can be accessed via POP3, IMAP, SMAP, and HTTP.
 * A faxmail gateway that forwards E-mail messages via fax.
 * Courier includes a mailing list manager.
 * PAM, LDAP, PostgreSQL, or MySQL authentication.
 * Authenticated SMTP.
 * Integrated mail filtering.