This strategy has been found to be a viable solution to a variety of email issues that arise with cellphones. This may be the solution if the problem is caused by wrong email account settings. Before you do so, double-check that your whole email archive is available on your email provider's website (webmail). You may check your webmail account on your computer before leaving the house. After you've double-checked that your emails are still there, follow these steps:

1. On your iPhone 6s Plus, go to Settings.
2. Select Accounts & Passwords from the drop-down menu.
3. Select the email account you wish to delete.
4. To delete an account, tap Delete Account.
5. To confirm, press Delete email if asked.
6. Restart your iPhone if necessary.
7. Then re-enter your email address.

You can choose to have your email account set up automatically or manually. If you choose the latter, double-check that you have all of the relevant information, such as your email account credentials and server settings. You may get this information from your email provider. If you don't want to do anything manually, go with automated setup.

Go to Settings-> Accounts and Passwords-> Add Account-> Exchange to do so. Then connect to the Exchange server and sync your contents by entering your email address and account description.