That makes it a lot harder, but there still may be a way. What is the email setup there? Can you create additional email accounts under your domain? If so, can you also enter forwarding information? And does the ISP have the cgi version of PHP available also?
Secondly, do you have the ability to create crontab entries for yourself? A second possibility, if the above line of questioning leads nowhere, is to set up a separate email box and then fire off a php script at regular intervals to read that email and respond to it.