Hey Martin,
For other users to be able to read InfoPath forms you send them they will
need the following
1: The InfoPath 2003 client installed, this is the same client you used to
create the form
2: They will also need access to the template file you created. e.g. when
you create a form and publish it an .xsn file is created. An .xsn is purely
the template file when you fill out a form and save it (or in your case email
it) you email a .xml file. This xml file maintains a reference to the
original template that you created. So your template will need to be either
published to a sharepoint library, shared location or registerd on the users
machine.

Hope this helps

Cheers
Ben Walters