If you've set up more than one email account on your mobile phone, you need to select a default email account. When you write email, they'll automatically be sent from your default email account.
Find "Default Account"
Select default email account
Tap Mail, Contacts, Calendars.
Tap Default Account.
Return to the home screen
Tap the required email account until it has been selected as the default email account.
Tap the Home key to return to the home screen.