After you make sure your Browser enabled Documents are set to Open as a Web Page under Advanced settings of your document library.
If your SharePoint is not configured right, you will see
First, you need to make sure your server has Form Service turned on
Then you need to make sure this feature is active on the site collection level.
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.