There is a known issue where Outlook 2016 cannot log into a second mailbox.
A student employee logs in with their UNT account (UNT\\euid0123). when they open Outlook, it is automatically configured to connect over to their UNT mailbox (FirstName.LastName@unt.edu).
If the student employee then tries to connect to their student mailbox (FirstName.LastName@my.unt.edu) in the same Outlook profile, they will get an error.
Start CAS Remote (ScreenConnect) on the affected computer and then call the CAS ITS Service Desk while you are at the affected computer. We can apply a group policy to your account that will resolve this issue.