How to set the default email account on my Apple iPhone 6S
If you have multiple email accounts on your Apple iPhone 6S, you can set one as the default account for sending email.
1. Touch Settings.
2. Scroll to and touch Mail, Contacts, Calendars.
3. Scroll to and touch Default Account.
4. Touch the desired account.
5. Touch Mail…
6. The default email account has been set.
Note: You can still send email from other accounts; you can choose the "from" account when composing individual messages.