MailUtil.php error: failed to open stream

If you are getting the error:

File:       /opt/fogbugz/Website/MailUtil.php
Line:     45
Error:     require_once(Mail.php) [<a href='function.require-once'>function.require-once</a>]: failed to open stream: No such file or directory

during the installation process, this means that the Pear Mail module did not get properly installed for some reason.

Run the command:

pear install Mail

That should fix the problem.  Depending on permissions, you may have to put a sudo in front of the command, as illustrated by this comic.