read this somewhere.... going to try it
"1. Leave your multi-account as-is set up on your iPhone, but remove your preferred default e-mail address from the list
2. Set up a new e-mail account with IMAP access (i.e. Gmail or whatever)
3. Create a new IMAP e-mail account on your iPhone/iPad with the details of this new account for the IMAP (receive) settings, but with your original e-mail account's SMTP (send) settings. Also, set the "Email" field in the account setup to be your preferred default e-mail address.
4. Set this new mail account as the default in Settings > Mail, Contacts, Calendars
Now, when you create a new message, you should see that your default "from" address is correct - since it is the only choice for the new mail account you set up in iOS. As long as the SMTP settings are correct, it doesn't matter that your IMAP settings don't match.
You should still be able to select any other "from" address as well, which will use your original multi-account settings, which are also correct.
All of your received mail will come in through your original multi-account inbox, and your new account's inbox will likely stay empty.
It seems like I'm forgetting a step here, but this should give you the right idea for a workaround if it isn't exactly right. Feel free to add corrections if you find them."