- Proper handling (and generation) of threading headers like "References" and "In-Reply-To". And, hence, a threaded view of my mail.
- The ability to just display the text/plain version of the mail. Just about every webmail I've used insists on trying to render the HTML version - and I don't want to see that.
- Virtual folders. The ability to put a mail into multiple folders. Maybe this can be achieved using tags (a bit like how Gmail does it).
- Multiple identities. I send mail that needs to appear to come from many different email addresses. Each different identity needs to potentially have different settings for From:, Reply-To:, SMTP server and signature (and probably other things). Identity should be able to be set automatically according to various attributes of the mail that I'm replying to (from:, to:, the current mailbox, etc) but I need to be able to override it when I'm composing the mail.
I guess I'm really looking for a web-based version of mutt :)
Oh, and in my opinion, Mail::Box is looking a bit old and over-complex. If I was you I'd look at the modules from the Perl Email Project.
"The first rule of Perl club is you do not talk about
-- Chip Salzenberg