Not Mozilla’s but Thunderbird’s appeals were seen as mostly positive (and independent of Mozilla).

> Same applies to Mozilla.

Not Thunderbird. It is just a poor abandoned child.

Mozilla, maybe.

What ongoing work does an email client need, though, besides fixing bugs and very occasionally adding new login protocols?

If you ever try to write a email client, you will immediately realize how difficult, if not impossible, to fix all the bugs for a email client. It is a multi-different-protocol-version-async-client-handling-same-database-with-thousands-of-race-condition backwards compatibility nightmare.

Writing a email client with support of just up-to-date protocols and assume it is the single client that will operate that account is trivial, write one that covers all corner cases is a totally different story.

I don't know about the rest, but surely the race conditions are the fault of whoever designed the concurrency part. An email client does not inherently have race conditions.

> assume it is the single client that will operate that account

You are still making wild assumption without actually thinking about what means to writing an email client.

OK? Sure would be nice to hear why having a second email client talk to the remote server introduces race conditions on the local client (EDIT: that is, race conditions that are the local client's responsibility).

Adding a mobile version, MS Exchange integration, supporting OAuth2 login, refreshing the UI periodically because otherwise everyone whines about how "dated" it looks.

Their RSS could use some improvements, but the same could be said about all of them.