Why is outgoing email not being sent by the default email account!

I have setup 2 email accounts on the Iphone 6, 1 for a yahoo account and 1 for an exchange account and set the defaults to the exchange account but when email is sent from the phone, it goes out the yahoo account.

Please Help

When you're sending outgoing mail, tap your email address & you'll have the option to choose which address you want to send the email from.

Go to Settings - Mail, Contacts, Calendars - Default Account, and then verify that your exchange account is set as the default mail account.