After several more or less happy years with Thunderbird as my default email program, I have now made the move “back” — I am now using MS Outlook. There were some mysterious crashing problems with Thunderbird: after the latest 3.x.x update, the damned program would not close without crashing. But the ultimate reason was support for synchronization and compatibility. Outlook calendar is “better” from my perspective, due to the support it receives from third parties: it is easier to make it sync with my various mobile devices and online calendars. There was the Lightning extension for Thunderbird, but even together, they just could not compete with the flexibility and range of features in Outlook (I am using Office 2007).
One obvious issue, though, that I have found out in Outlook. It does not include option to send automatically a “blind carbon-copy” (bcc) to a message of your choice — this is something that I need, because of the multiple email systems that need to keep in sync). I tried various solutions to come around this, including installing and editing some Visual Basic code, but to no functional result. There were several commercial add-ons that promised to do the job, but really: paying 20-30 dollars/euros just to get auto-bcc feature? Finally I did find this free component, Alan McGrath’s AutoBCC:
http://www.mcgrathtechnology.com/addins
This seems to work. You input the bcc: address into Tools > Options > AutoBCC settings (and it does not appear in the actual bcc field of your message that confused me a bit), but it seems to send the bcc copy nevertheless just fine. Hope it will work for you, too.
frans goes blog 
What was actually “fun” to realize was that it is almost impossible to move your emails (incl. folder structure, your filters/rules) from Thunderbird to Outlook — it is much easier to move from Outlook to Thunderbird. That much about the distinction between the “open” and “closed” software worlds…
“…it is much easier to move from Outlook to Thunderbird.”
Probably because you’re the only one who’s ever done the opposite! 😉
Heh, might be — or can it be that I am the only one who has been successful in it?? (Actually, you can find stories of people trying to carry out the Thunderbird > Outlook transition from the Internet, and yes, it is really hard for most of them.)
Running Office 2010 beta right now with the respective Outlook version, while running Thunderbird on my netbook (performance). I like the new Outlook quite a bit. In particular, IMAP seems to be implemented much better and much more performant. Not sure about the default BCC, but I’m wondering why you wouldn’t try to use e.g. IMAP to keep all your accounts in sync?
Uwe, thanks for the tip, I am going to test Office 2010 also some of these days. About using IMAP to keep all accounts in sync — the bottleneck is our university’s mail system that does not keep up with my hundred–several hundreds of emails per day. I need to keep on wiping the IMAP account clean all the time, not to run out of disk space (and have off-site synced archives to for search and verification purposes). Some day I will probably only use a single Gmail account, but our regulations do not allow it at the moment.