I have a new Exchange 2016 server. OWA works perfectly. iPhones and Androids using ActiveSync work perfectly. Outlook clients on the local domain work perfectly. All and send and receive just fine. My Digicert certificate displays correctly for HTTPS.
The issue I am having is with the few remaining POP users. They can receive email just fine, but can't send. The issue is that the SMTP server won't accept anyone's credentials. Regardless of the email client, it always comes back and asks for the password again.
I have followed all of the steps here and triple-checked: https://technet.microsoft.com/en-us/library/gg298947(v=exchg.160).aspx
When I sign in to OWA and go to options, I see the SMTP settings displayed pointing to port 587.
What am I missing?