Hi Michael and Graham,
I was originally a (very happy) Eudora user. I got along with Apple Mail for awhile until Apple killed the Letterbox plugins (which allowed creating a very simple single line per email interface similar to Eudora).
What I use now and love is Mailmate. Mailmate looks quite a bit like Eudora and is almost as geeky in its advanced settings. In Mailmate, one writes in Markdown, the same easy markup language used here on TidBITS talk. Benny, the developer is a real person who answers his emails and really cares about his users and is regularly updating the application.
There’s no constant pressure to buy a new version. Mailmate is still on 1.12 while most email vendors would be on version 7.93 if they had been around as long as Mailmate.
Try Mailmate, I think you’ll like it.
PS. Mailmate is all IMAP with no local database, so archives would have to be kept in Apple Mail (what I do), which is fine as the daily interaction with email all happens via IMAP these days anyway and it’s very easy to make archives from IMAP within Apple Mail. This is a very smooth process.