Outlook 2016 keeps asking for credentials

They are on a SBS 2011 Server using Exchange. Outlook 2016. The Accountant's Outlook will occasionally pop up and ask for credentials. It is a laptop but connected with a cable. I have deleted everything in the Credentials Manager. Can't figure out what is causing this popup.
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Did the laptop have WiFi enabled while connected with a cable?  If yes then test for a few days by disabling the WiFi.

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LockDown32OwnerAuthor Commented:
I believe it did yes but early on in this project we disabled the wireless and went cabled. Still doing it.
Any chance of deleting then recreating the email profile?
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Jakob DigranesSenior ConsultantCommented:
This might be autodiscover - since it pops up occasionally. Run through the test email autoconfiguration (https://www.codetwo.com/admins-blog/how-to-quickly-verify-if-autodiscover-is-working/)
also - are you running Lync or Skype for business also?
LockDown32OwnerAuthor Commented:
Every time someone mentions autodiscover I cringe :) The test passed but.... I swear this one computer with the problem isn't following the correct autodiscover. I changed the MX record about a week ago and if memory serves, for some reason, the autodiscover wanted to use what I changed the MX record to.

   The test showed the correct autodiscover. When time permits I'll delete the Outlook profile and recreate it just to see when the autodiscover points it. It is 2016 which I hear is even more wrapped around the autodiscover.
LockDown32OwnerAuthor Commented:
I just watched the Autodiscovery. It "discovered" an IMAP account where the MX record is pointing and the smtp server was. It completely missed the fact that they are on a LAN with an Exchange Server.

Outlook 2013 doesn't do this. Seems to be only Outlook 2016 and Outlook 2016 was fine until I chanhed the MX record.....
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