If you have multiple email accounts set up on your iPhone or iPad, the Mail app will automatically send a new email from your default email account. To change that behavior you can set default email account or select sending FROM email address.
- Go to Settings > Mail, Contacts, Calendars, then tap Default account:
- Tap on the account to select it as the Default account:
- Launch Mail app > tap on Compose button in the right hand corner of the main mailbox scren
- Tap on From field > select email account