When pressing send the data gathered through that form has to be send by eMail.

This eMail has to be encrypted.

Because I am not supposed to change the application I am looking for a tool that sits in between the generation of the eMail and the MTA.

A search on Google showed pgpsendmail quite a few times. Is this tool any good?

I don't know about that tool, but I don't see any problem in simply calling GPG after you've generated the email body to encrypt (and perhaps sign) that, whereafter you can pass it to the MTA. Tobias