Open Outlook 365 (or Outlook 2016):
Click on FIle in the Left upper corner:
A window opens with your current E-Mail accounts.
Now click on the email account you want to check/Adjust and then on Change…:
The page that appears should look like this:
Double-check that the field Email Address contains the correct email address.
(Double-check the spelling!!)
Next pay attention to the Incoming and Outgoing Mail Server. Both should be set to: mail.yourwebsite.com
(replace yourwebsite.com with your actual website/domain name.
It normally is the part AFTER the @ sign in your email address.
So, if your email address is firstname.lastname@example.org then Incoming and Outgoing mail server should be: mail.yourwebsite.com).
Next, click on More Settings…