I stand corrected. Although Sylpheed looks superficially like the old T-bird type of interface it isn't a fork but Interlink is. Sylpheed claims to offer news but the version in the Debian repository doesn't offer it in account setup although the online manual suggests it does; so strike that one off the list.
Given the extent to which people here beef about Outlook I guess there are plenty of people still using that client. However there's scope for a good deal of improvement in any client that I've seen. What's needed is to look beyond just message handling.
Accumulated messages - in- and out-bound can represent a lot information with long term value. That applies to project work IME. Looking back to pre-email days, it would apply equally to case work. But that information is hidden away in a fairly unhelpful interface. No wonder we hear of people storing huge amounts of mail in their inbox or, even worse, their deleted folder.
In a well-run traditional paper-based office documents including incoming mail and carbons of outgoing mail would be properly filed by a filing clerk with all sorts of other, related material. We ought to be able to do as well as that by automating the filing clerk.
In practice if I'm working on a project I might have a desktop folder containing documents I've written, stuff I've researched from the web and material I've been emailed from collaborators. However if I want to read what was in those emails other than those saved attachments I've got to go scrabbling about in the email client's rudimentary filing system UI. I could copy it them out into a text document in the folder but then that has to be maintained as fresh emails are sent and received.
What would really be useful would be to not only set up a folder structure in the email client but also to link the relevant folders into the desktop folder along with all the other material and be able to click on the email threads within them and get them presented in a similar way to comments here. And wouldn't it be useful to add a personal note against an email, one that's not going to be quoted in a reply or included if I forward the email to someone else?
And, of course, what applies to email could also apply to any other form of communication the application could handle. We've only scratched the surface of what an "email" client could and should really do.