<quote>
Copies of this library code used to exist in other tarballs: Courier,
Courier-IMAP, and SqWebMail. Building and installing any of these packages
would've automatically installed this authentication code.
The authentication library is now a separate, standalone package. This
authentication library must now be installed, separately, before upgrading
to the following builds (or if installing them for the first time):
Courier 0.48, Courier-IMAP 4.0, and SqWebMail 5.0. See NEWS for more information.
</quote>
http://www.courier-mta.org/authlib/
c16332dab7