iPhone.cz - fórum

There is a solution here that will allow that to work, you just need to have the Gmail account setup on the iPhone as well.

Go to Gmail web interface, then to Settings -> Forward Account and add your iCloud email to forward all your emails to. I also chose to mark emails forwarded to read, since I don't use the web interface much.

Setup Gmail on your iPhone like you normally would (skip if it's already on there). But under Settings, do not have it sync emails.

Setup your iCloud account to send using your Gmail address.

Setting -> Mail, Contacts, Calendar -> iCloud -> Account -> Advanced Mail -> outgoing mail server -> TURN iCloud SMTP server OFF and turn other SMTP servers on, choosing your smtp.gmail.com settings associated to the appropriate gmail account. HIT DONE.
A few things about this setup... When you go to reply to a message, it will show as if it's sending from your iCloud account, it does not. It is just using the settings you just set that are associated with the iCloud account.

This only works if you don't really use your iCloud account and you only need to reply via one Gmail account. I had several Gmail accounts, but only respond from one, so this is not a big issue for me.