I have to agree with many who've commented here.  The author doesn't list a single problem with current email clients, except maybe that he hints at wanting one with keyboard accessibility rather than requiring a mouse.  Either he has no idea what's available (in which case, he should search first), or he hasn't really thought about what he wants in an email client.<p>Regarding what he considers obvious:
- Cocoa? I have no idea why Cocoa would be obvious.  Surely an email client for developers would need to run on at least Windows, Linux and Mac OSX since there are enough developers that use each of these.
- IMAP only?  Good idea, but that's really the easiest part of the problem.  I'd say defer this to fetchmail to start with and just read from a local directory.  Add actual IMAP support once you've figured out the UI.  That's the real hard part.
- twitter account?  yeah.  'nuff said.<p>The biggest problem with email today is not that a client doesn't exist to satisfy your needs.  Every developer I've met does in fact have at least one email client that satisfies all their requirements.  For some it's thunderbird, for others it's gnus or mutt or pine, others like gmail, still others like Mail.app, and believe it or not, there are actually developers who like MS Outlook or Entourage.  No, the biggest problem with email is that trust isn't yet built into it.  PGP/GPG/DKIM/Authenticated SMTP, and others exist, but the majority of email users have no idea what these are or how to use them, and unless every email user in the world switches to using some kind of trusted authentication system, email will stay the way it is today.