Feature suggestion: It would be awesome to have multiple message lists available. That is, multiple copies of the mailbox list (if open) + message list on the left (with a separate columns header for each message list if possible, preferably toggleable), with a single preview pane on the right. You should be able to drag the message list separators to resize them. Things like individual sorting/threading options for each would be nice.
I'd love to be able to have, say, 3 message lists open: One sized to about 60% vertical height for work email, one sized to 25% vertical height for personal email, and one sized to 15% vertical height for non-important work email (automated notifications and such.)
I currently manage this by using rules to set background colors for different categories of work/personal mail (viewed as a unified inbox of two IMAP accounts), and I don't even bother displaying the non-important work mail since the volume is too great to mix it in with my inboxes, but if I could have a non-intrusive view of it taking just a bit of space, that'd be nice. This, of course, results in an interleaved view; what I want is a non-interleaved view of multiple message lists, each sorted separately.
Oh, and another feature suggestion, as if I wasn't already asking too much: A From/To column. Or, just the ability in the "From" column to have it display like "To: " if it's a sent message (or however else this can be matched if possible.) I like to display my sent mail with my inboxes as well, which I also color code. But it'd be nice to be able to see the recipient of sent mail in the same "From" column, rather than just see that it's from me.
I could accomplish this through a couple of different hacks, such as having a script on my IMAP server copy sent mail into an archival mailbox for the original, then rewriting the From to "To: xxx" in the main sent mailbox. Or perhaps creating a Smart Mailbox and using an Applescript to do the same. But it'd be nice to just have a simple way to do it.