Change default email account address in iphone and ipad

In ios5 and above you can change the default email account – allowing you to send emails from a specified account.

To do this goto:

settings > mail, calendars and contacts
scroll down and find ‘Default Account’
select it, and you are presented with a list of your configured mail accounts to pick from.

pick your desired default account.
Your emails will now automatically have this accounts email address as the ‘from’ field