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.

 Set default email account:

  1. Go to Settings > Mail, Contacts, Calendars, then tap Default account:
    Settings
  2. Tap on the account to select it as the Default account:
    Settings

    Select FROM email address:

    1. Launch Mail app > tap on Compose button in the right hand corner of the main mailbox screnCompose
    2. Tap on From field > select email account
      settings