I just noticed that the pear configuration is wrong and the packages are installed in the path we use to compile the different components. In the case of Mail_Queue, it's installed at
/bitnami/lamp71stack-linux-x64/output/php/lib/php/Mail/Queue.php so you can use it by adding this line in the php file
I tested that and it returns an error message so I just passed this information to our Engineering team and they will check that.
bitnami@bitnami-lampstack-dm-99ac:~$ php example.php
PHP Fatal error: Cannot unset $this in /bitnami/lamp71stack-linux-x64/output/php/lib/php/Mail/Queue.php on line 269
As this version was published in 2011, it is probably not supported in PHP 7.x. I suggest you ask in any specialized forum about PHP or PEAR to get more information about this.
Sorry for the inconvenience