/usr/lib/libcourierauthsaslclient.so is in courier-authlib-dev 0.68.0-4build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libcourierauthsaslclient.so.0.0.0
/usr/lib/libcourierauthsaslclient.so.0.0.0