I use Thunderbird for all my mail needs (except Gmail as it doesn't do IMAP) and there are several capabilities within it's filtering that I wish it had:
I cope without these, but having these would save so much time and make my life, particularily at work, so much easier.
Bugs/RFEs have already been logged for each of these features, but time will tell if they are ever integrated.
With this in mind, I was quite delighted to read today that Eudora are dumping their proprietry email client in favour of using the Thunderbird code. From the press release:
... Future versions of Eudora will be free and open source, while retaining Eudora's uniquely rich feature set and productivity enhancements. QUALCOMM and Mozilla will each participate in, and continue to foster development communities based around the open source Mozilla project, with a view to enhancing the capabilities and ease of use of both Eudora and Thunderbird. ...
Eudora currently has the ability to do 3 of the 4 things I want and if the above statement is anything to go by, I'd expect these features to appear in the open source version of Eudora and accordingly, Thunderbird.