mail/courier-imap 5.2.10p1 OpenBSD

Courier-IMAP is a server that provides IMAP access to Maildirs mailboxes. This IMAP server does NOT handle traditional mailbox files (/var/spool/mail, and derivatives), it was written for the specific purpose of providing IMAP access to Maildirs.

The Courier-IMAP implements some extensions to the basic Maildir mail storage format in order to support some additional features, such as folders and "soft" quotas. This is done in a way that's backwards compatible with the standard Maildir users (although, of course, they will ignore the additional features). Other software that understands and uses the same Maildir extensions is SqWebMail, and maildrop.

"Maildir" is a directory-based mail storage format originally introduced in the Qmail mail server, and adopted as an alternative mail storage format by both Exim and Postfix. Courier-IMAP is popular on Qmail/Exim/Postfix sites that are configured to use maildirs. The primary advantage of maildirs is that multiple applications can access the same maildir simultaneously without requiring any kind of locking whatsoever. It is a faster and more efficient way to store mail. It works particularly well over NFS, which has a long history of locking-related woes.

It is part of the Courier mail server (http://www.courier-mta.org) but can be configured as a standalone IMAP server.

Features include an extremely small memory footprint, abstract authentication modules (e.g. /etc/passwd, LDAP, PAM), virtual mailbox support, resource controls, SSL/TLS, and IPv6 support.