As far as I'm concerned, Orchestra's excellent email application Mailbox - now owned and maintained by Dropbox- needs only a few little touches to become the ultimate mobile email client. With today's not-to-be-missed refresh, Mailbox has become the closest thing to email nirvana, at least for me.
Didn't use Mailbox because it wouldn't let you customize signatures on a per-account basis? Solved!
The annoying inability to search every message in your Gmail account? That too! And as a bonus, Mailbox can now open links in Google's Chrome iOS browser...