If you've set up more than one email account on your mobile phone, you can select a default email account. When you send an email from one of your mobile phone's applications, the default account will be used unless you're asked to select an account before sending the email.
Slide your finger upwards on the screen.
Slide your finger right starting form the left side of the screen.
Tap the settings icon.
Tap the menu icon.
Tap Set default account.
Tap the required account.
Tap DONE to select the email account as your default account.