Seth reported problems with sending through their IMAP account do to the email in the header being different. This was mentioned in the comments section of my Gmail, IMAP, and iPhones – Part 2 – The Tutorial post.
I’m assuming that some ISPs allow this and some don’t. Fortunately, as Jared mentioned, there is an easy way around this by using Gmail as your outgoing SMTP server. This also has the advantage that you don‚Äôt need to CC yourself on all emails, and as such you might want to set it up this way anyway. However, it has the disadvantage that sent mail is not also stored on your IMAP account‚Äîthis is not a big deal as it is stored in gmail.
I’ve updated my old post with this option.