Courier can function as an intermediate mail relay, between an internal LAN and the Internet, or perform final delivery to mailboxes.

Courier uses maildirs as its native storage format and can also deliver mail to legacy mailbox files.

Courier can provide mail services for regular operating system accounts.

Courier-IMAP is a particularly popular combination with Qmail, Exim, and Postfix servers that are configured to use maildirs.

Courier's source compiles on most POSIX-based operating systems based on Linux and BSD-derived kernels.