I made the switch to Thunderbird a week after switching to Firefox. The main reason was built in spam blocking (which I haven't really had a chance to use yet).
The first thing I noticed about switching is that amazingly, the import process from Express to Thunderbird worked perfectly! I had 4 different accounts in Express pulling mail from 3 different email servers and with the exception of passwords, all my messages were properly sorted and each account was transferred perfectly! Wow!
So far so good - spam blocking is next!
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