If you keep SquirrelMail behind a password-protected directory on your web
server and if PHP has access to the username and password, this plugin will
bypass the login screen and use the provided username/password.

If this plugin enabled and the information isn't present, it doesn't crash
or produce error messages. It just doesn't automatically log you in. If the
user/pass information does not work for the IMAP server, this still displays
the login form, just in case the web server password and IMAP password are
not synchronized.