Mailhandler module requires extensions to PHP in order to interface with different mail servers.

If you don't have these extension installed you may get errors like

Fatal error: Call to undefined function imap_open()

IMAP on Debian/Ubuntu

If you use a Debian/Ubuntu system then you will need the php5-imap extension. To install this and get Apache to recognize it requires two steps:

sudo apt-get install php5-imap
sudo php5enmod imap
sudo /etc/init.d/apache2 restart

This installs the PHP module and should configure it and then restarts Apache. Note that your website will go down while you restart but it should only take about 30 seconds for it to come back.