There are many (some of which fall into the maintenance realm and too involved to discuss here).

The most notable from my perspective are:

- Eudora's easy setup of an account without having to go online.

- Easy moving of data/accounts from one storage location to another (Thunderbird's Command Line 'profilemanager' is an abomination—an afterthought of the worst order.)

- Eudora could dock the message window, this is not possible in Thunderbird (why the hell not?Thunderbird's ergonomics are horrible).

- Adding and removing quotes levels in Eudora is a snack, it's difficult and time consuming in Thunderbird.

- There are other Eudora features both at a menu level and as part of the UI that make it easier to use than Thunderbird (it'd be best to illustrate these with a few images but obviously I can't do that here).

- Resending messages where faults have occurred is a pain. The other day I had a problem and I wanted to resend the only remaining copy of a failed message I had was a BCC to nyself. There was no simple way to reconfigure the message's headers from within Thunderbird and do so cleanly so i had to make a new mail folder and copy the message there. I then had to dig deep into Thunderbird's Window's directories then load that message into a text editor and manually edit the header (what a performance-T'Bird just isn't up to anything out of the ordinary as Mozilla's developers have straightjacketed much of the code). Heavens knows why, Mozilla's developers NEVER explain why they do something, nor do they respond adequately when users complain. Mozilla's lack of consideration for its users is one of the single biggest problems with Thunderbird.

- Thunderbird's documentation sucks (exactly why would require me to write a full page which I can't do here).

- Essays could be written on Thunderbird's problems and I'm at a loss why many more usrs aren't complaining (of course that's the real problem—too many people are prepared to just put up with shit software).

Clearly, I'd like to add more but likely it wouldn't contribute much extra at this point.

Let me finish by saying that Eudora wasn't perfect by any stretch and it wasn't helped by Qualcomm running the product down over the last decade of its life.

The real issue is that whilst Eudora has been dead for well over a decade and that many of us are still mourning its loss tells us much about Thunderbird—specifically its lack of development in the intervening time since Eudora demise.

There are many things that need to be done to Thunderbird such as separating the browser and editor out of the main mail package and or allowing the integration of an alternative browser, same goes for Thunderbird's brain-dead and very buggy editor (it's had memory leaks for years where one's text would simply vanish, there's been no attempt to fix this bug and it's a waste of time arguing with Mozilla about it).

Mozilla developers don't fix things, instead they add facy baubles instead of addressing essential utilitarian matters that are broken.

Of course, writing new code is more interesting for developers than bug fixes, and unfortunatly that's the penalty users have to endure when code is 'free'.

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Edit: FYI https://news.ycombinator.com/item?id=33875560

If you want to piece more of this Eudora/Thunderbird story together then don't just rely on my comment but also read the story and other comments.