If you've got multiple email accounts set up on your iPhone or iPad, the Mail app will automatically reply from whatever account an email was sent to. But what about when you're composing a new email? If you've got one account you use more than others, you can set that as your default account for composing emails. If you ever want to send from another account, you can still do that too.
How to set a default email account on your iPhone and iPad
Launch the Settings app from the Home screen of your iPhone or iPad.
Now tap on Mail, Contacts, Calendar.
Scroll down the page quite a ways until you see an option for Default Account. Tap on it.
Tap on the account you'd like to be the default account for composing mail.
Tap the Home button to return to the Home screen. That's all there is to it. Your iPhone or iPad will now remember which account is the default for composing emails.
How to switch between emails accounts on your iPhone and iPad
Launch the Mail app from the Home screen of your iPhone or iPad.
Tap on the Compose button in the bottom right hand corner of the main mailbox screen.
You'll see a section on the top that shows what account the email is currently coming from. Tap on it once and you'll notice the From section will now jump into it's own field.
Tap on the From field one more time in order to change the email account you're sending from. You'll now see a ticker that allows you to switch between accounts. Simply tap on the one you'd like to choose.
That's all there is to it. Your new mail message will now be sent from the account you've selected.
You'll also notice that if you've configured different email signatures for each account that the Mail app will automatically change the signature to correspond from the account you've chosen to end from.