Pegasus Mail
pmail.com
pmail.com
I know the names of a few, but I haven't tried them: Sylpheed, Claws, Evolution (cross-platform?), Kmail (cross-platform?), N1 by Nylon, MailPile (FLOSS?) ...
[0] See this discussion, for example: https://mail.mozilla.org/pipermail/tb-planning/2017-March/00...
For example, if they move the content of the RSS feeds into a database (They'll keep RSS feeds .. right?) my two-line bash script for automatically downloading videos from links contained in one feed wouldn't work anymore ;)
There were services where humans actually sometimes tried to manually route an email?
EDIT: Ah, http://www.faqs.org/rfcs/rfc2821.html states it should still be so.
Sigh.
In the documentation for MailMate, I don't see the kind of filtering that I use with Pegasus Mail. This is one of the things I miss most in all other clients: I prefer to collect the mails in my inbox, without filtering. When I close the inbox, Pegasus runs a large ruleset with hundreds of rules, moving all the read mail to archive folders and leaving the unread mail in the inbox. I've not yet seen any other client capable of doing this.
Portability issues aside, that's a questionable foundation.
If I recall things right this had a built in email list management. I had acquired a CSV of all the student and staff addresses. While I was goofing around with this I accidentally sent out a to a really strange email to everybody (but it kind of looked like it was addressed to them because everybody was in BCC). Next day at school was terrible.
But it was buggy and not very friendly, so we migrated to Eudora, and indeed users never looked back.
I remember how much I disliked using it in Uni!!
The English language has actually changed during Pegasus' lifetime.
Don't most email clients have that? That's how I send emails on Gmail and Outlook.