Opera

In order to use "The Bat!" as an external e-mail client when clicking "mailto" links in Opera, open the "Tools" menu, and go to Preferences > Advanced > Programs. Double-click the "mailto" protocol and select "Open with other application". Use the "Choose" button to point Opera to the location of "thebat.exe".

Add this as a command line argument behind the path to "thebat.exe":

mailto:%t?subject=%s&body=%m

This will make Opera fill in the "To:", "Subject:" and Body fields when you click a "mailto:" link to compose a new mail.

The resulting command in the "Use external client" field should be something similar to this:

C:\Program files\The Bat!\thebat.exe mailto:%t?subject=%s&body=%m

Note that this is just an example, and that you should replace this path with the correct path to "thebat.exe".

(This has been tested with The Bat! 1.53d.)