tl;dr: I have several email addresses on the same Mail account, iOS defaults to the wrong one as the sender, and I can't figure out how to change that.
I set up my email accounts on my Mac. In Mac OS X Mail's Accounts prefs, it's possible to list several email addresses for an account in a comma-delimited list. I do this, and it works well on the Mac. In the Composing prefs in Mail, I set Send new messages from to my primary email address, and that's the one that gets used by default.
On iOS, Mail uses a different email address in the From and Reply-To headers. (It's an address I only use for email to/from Yahoo groups). I haven't been able to find anything like Mac OS X Mail's Send new messages from setting, so I don't know how I can change this. As a result, email from iOS defaults to having a return address that I really don't want it to have. I only get the preferred address if I remember to change it, every single time.
Deleting the address isn't an option-- I do actually use it, sometimes.