If they enter their normal domain credentials can they then email that user?
Also, forgot to ask, are they using Outlook or OWA when emailing this user? or Both?
They are signed onto the domain using the Outlook exchange email and when anyone emails this one particular user frmo outlook, they are prompted for log in credentials to the OWA client. If they cancel, this particular user still gets the email though.
Their profile in the ADUC is exactly like everyone else’s – no restrictions, that’s what’s so strange.
This is such an unusual issue I would get help from Microsoft for this one.