I've been using Thunderbird for a couple of years on my primary email account, have had no problems with it (fwiw the mailserver is MS Exchange, which has for some time been supporting IMAP access perfectly satisfactorily). I've not yet managed to get it to play well with some accounts on a different ISP (functional, but s l o w), but that's not Tbird's fault.
Before I retired, my PC at work had MS Outlook (the proper one, not OE) 2007, which was acceptable (well, had to be, there was no other choice), but I haven't used the 2010 version; it's a possible option if your ISP runs MS Exchange and you're willing to spring for the price of an MS Office edition that includes it. I think a trial version is downloadable.
Milo Minderbinder:
... trying to set out multiple outgoing SMTP accounts is simply a non-intuitive nightmare ...
Have to agree on that one.
Personally I think the new open source version of Eudora may be better.
Used Eudora years ago in the dial-up era. Thanks for the tip.
WGW