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.