When your using an email link you need to make sure that the link uses the
full InfoPath URL or appends some query parameters.
If your URL looks like http://server/documentlib/myform.xml you would need
to append the openin parameter as follows:
You can also use the full URL to InfoPath as follows:
This will force InfoPath server to open the form in the brower.
Take a look at my blog about form invocation for more information.
Software Development Engineer
InfoPath Forms Server