Opera Mail is just Opera 12 with some tweaks to hide the browser stuff (which also introduced some bugs etc.). It was never finished and is no longer being worked on (besides the recent security update, which only applies to win and mac as there's no official *nix build). The version that's in Opera 12 works better.
You can still use Opera 12 for mail. In the advanced preferences under "programs and features", just add/set the http and https protocols to open in the Opera browser so links in emails open in the browser.
You can start Opera 12 with the -pd "path to directory" to make it use a specific profile folder if you want. You can put your old mail folder, contacts.adr, operaprefs.ini, wand.dat and opcert6.dat there. Or, just let it use the default .opera profile folder if the upgrade to the new Opera didn't take it over.
Disable updates to Opera 12 if it tries to force you to upgrade to the new Opera.
