Like I said, much as I like Eudora – especially it's filtering, personalities and stationery capabilities (plus, it must be said: being now unsupported I'm not constantly bombarded with updates/new versions to go through learning curves again) – I wouldn't recommend it for a new user.
There are cracks beginning to show. For example: it cannot properly handle the UTF-8 character set which is becoming ubiquitous (thanks Apple). There is a workaround available (for the Windows version of Eudora) but for me the cure is worse than the disease.
Pandora has a good reputation among its users, many of them ex-Eudora users (Pandora was intended as a Eudora replacement).
I have no experience with modern Thunderbird, as I said. Nor any other client actually except Outlook Express many moons ago when it was foisted on me at the office. I can't believe anyone would want anything that ghastly. Don't know if it – or it's derivative – has improved since. I get the impression it hasn't.