Are you sure you don't need to use Opera composer to set the default email account settings for Opera to use? Also, the opera newsgroup may be a good source for information on special configurations for Opera.

There is a utility that works with Outlook and Opera that allows you to setup your default email client outside of Opera and Outlook:

http://www.mvps.org/sramesh2k/defaultmail.htm

and a utility for setting up the default browser when you have multiples:

http://www.mvps.org/sramesh2k/defaultbrowser.htm

Might be worth checking them out...