i know that the mail client is built-in the web browser. you need to enable the mail client in the preferences section of opera. i tried it, it was too much of a headache so i didn't want to deal with it. but hey, give it a try and post back on how it works for you please.
Under Tools -> Mail and chat accounts add the accounts you want to use. At work I have both my work email and my personal google accounts setup. The Opera mail is kind of like google with no folders with a lot of search and sort options.
It won't replace Thunderbird, I still have to use that sometimes to email graphical cut & pastes or if I want to make formatted emails but it does the job very well for 98% of my emailing needs at work and home.
Plus the convenience of having everything in the one pgm is great.
I have web-mail-1-2-3.xpi and hotmail-1-2-2.xpi add-ons installed in Thunderbird 2 and it reads my Hotmail and MSN e-mail accounts perfectly. (I couldn't get the hotmail-1-2-3.xpi released on 29 June to work, though). I am especially pleased because Hotmail and Outlook Express under Windows XP would not allow me to read a recently-created Hotmail account, only my original Hotmail account created many years ago, whereas the Thunderbird Hotmail add-on lets me access all of my Hotmail accounts.
i did run thunderbird with hotmail. but it had to be thunderbird-1.5 with its corresponding webmail add-ons. but after a while i get an error saying that there was negative vibes which made me use my work's email more than hotmail. now i use hotmail only to chat with friends, most of whom have just MSN messenger. I also get the same error message with TB-2 and its corresponding webmail extensions.