Notes on Thunderbird : I switched a while back from Outlook to Thunderbird at home. I have noticed that it is faster downloading email, but there are a few differences and minor issues that prevent me from making the switch at work as well.
1. Rules/filters : It does not convert the outlook rules into thunderbird filters when you convert everything else. You have to manually create all of the filters you need.
2. Preview mode : In Outlook, you can turn preview mode on or off for each folder, so the inbox is OFF but certain other folders could be ON by default. In Thunderbird preview is either on or off, and this is for ALL folders, no separate configuration per folder. F8 toggles it, so it isn't difficult to switch back and forth as needed, but I would still like to see it custom per folder.
3. Receiving mail - it doesn't always run the custom filters or the junk filters. I can't figure this one out, it seems random. Sometimes I get 30 messages and they all go into their assigned folders, with the junk mail getting so marked and moved into the junk folder, just as it should. Other times, it marks the junk but doesn't move it, and none of the other mail gets put where it belongs either - it all stays in the inbox. I can choose "Run Filters" and it all happens, but that turns an automatic thing into a manual one.
At work, I have around 25-30 filters that are really quite important. I don't want 300 emails in my inbox every morning! 200 of them are junk, but the rest are various logs and notices from many servers and such, information from the various web services I subscribe to, and other emails that need to be put into their proper folders.
This is the main problem - the intermittent nature of the filters working when getting email from the server. Once they get that working consistently, I'll switch to Thunderbird at work.