Hi,
If you are willing to using an SMS service that has an SMS API such as Clickatell or Twilio, I can pretty easily set this up for you.
All you will need to prepare is a text file that has the emails and phone numbers.
I will first add a hook in your Mail server that will trigger a PHP script every time an email is received. The script will look up into the text file to match the receiving email address with a phone number.
If the email is not listed, the script end. If the email is listed, the script will collect the associated phone number and use the SMS API to send to it an SMS (with a predefined message, or maybe an excerpt of the email based on your requirement).
The text file could be as simple as having in each line the email and phone number separated by a comma, that will make it very easy for you to maintain.
In the futur, to make it even easier, somebody could make a small and simple website where you can manage (add/edit/delete) all these pairs and store it in a database. In this case the script would simply need to be adjust to match the email with the list in the Database instead of the text file, nothing else.
Hope I gave you enough confidence that I can do it, and let me know if you have any questions !
PS: actually I have a question, I would be great if you could specify what mail server your are using and what operating system it's running on. Thanks.