How do I change my default "from" email address for the iOS Mail app?

Solution 1:

If you are creating a new message, then the "default" account will be used. If you are replying to a message in another account, that account will be used as the "from:" address.

It sounds to me like you want to reply to a message received on (say) Hotmail and have the message come "from" a new GMail address. This is not possible to do by default; you will need to change the "from" address manually.

Solution 2:

It seems to be related to the order that the email accounts were created into iOS. If you look at the order of the Accounts in Settings > Mail, Contacts, Calendars you will see what I mean.

I don't know of any way to reorder these, except by deleting them and adding them back in the order you want.