I've been working on a compatibility script
for Windows Live Hotmail (apparently the 2nd most popular webmail service, behind Yahoo! Mail
) in Opera. Overall, it has been smooth sailing - the only real differences between Opera and IE/Firefox involve some timing differences of multiple <script> onload events (there is no relevant detailed specification for this, so Opera isn't breaking any rules I know of).
I have found something in particular that Opera does really well: showing the "From" menu on the compose screen. In fact, Opera is noticably better at it than the competition:
(click for full resolution version, 276KB PNG)
Clearly, Microsoft has some more work to do, in order to ensure that their browser properly renders their webmail service