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 also implements basic web-based calendaring and scheduling services integrated in the webmail module.
Certain parts of Courier (the mail filtering engine, the webmail server and IMAP server) are also available as separate, smaller, packages that can be used with other mail servers.
- Implements SMTP extensions for mailing list management and spam filtering.
- Functions as an intermediate mail relay, relaying mail between an internal LAN and the Internet.
- Performs final delivery to mailboxes.
- Uses maildirs as its native mail storage format, but it can also deliver mail to legacy mailbox files as well.
- Configured by text files and Perl scripts.
- Dedicated web-based administration module.
Courier-IMAP’s features include:
- Extremely small footprint. On x86, Courier-IMAP’s main daemon is a fraction of the size of the UW-IMAP server, and has a greatly reduced memory footprint.
- Uses the Courier Authentication Library for authenticating mail accounts.
- Ability to restrict the maximum number of IMAP logins, and the maximum number of logins from the same IP address. This will prevent denial-of-service attacks where the attacker attempts to overload the server with multiple login connections.
- Shared folders. With additional server-side setup, folders can be shared between groups of accounts.
- IMAP over SSL. If OpenSSL or GnuTLS is installed, Courier-IMAP will, optionally, accept IMAP connections over SSL. The IMAP STARTTLS extension is also implemented.
- Authentication using SSL certificates.
- IPv6 support, if available.
- IMAP extensions that implement server-side sorting. This can be optionally disabled in environments that have limited resources.
- Courier-IMAP implements many popular IMAP protocol extensions that provide value-added functionality with most popular IMAP clients.
- Sorting and searching in non-English character sets.
- A compatible POP3 server is also included in the Courier-IMAP package, that uses the same authentication modules. The POP3 server can only access the INBOX, and cannot access any folders.
- An experimental mail access protocol, SMAP.
- IMAP/POP3 proxying. It is possible to distribute all mailboxes between multiple servers. A separate server (or a pool of servers) accepts all IMAP/POP3 connections, then connects to the right server based on the mailbox the connecting client is logging into.
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