In Opera 12, in the advanced settings, you should be able to goto "protocols" and add/edit the http and https protocols to open with the browser of your choice. Then, just use Opera as just a mail client and links in messages will load up in your browser when you click on them.
Mail in Opera 12 is better than in the standalone anyway as the standalone is unfinished and missing a bunch of settings.
The only downside to doing the protocol thing in Opera 12 is that iframes in feed messages will launch their sources in your browser instead of inside mail. If you don't use feeds though, you should be good to go.