Some customers were having issues with authenticating sending out emails from their Hosted Exchange accounts on their MAC devices.

Solution:

2nd tier support at Apple knowledgeable engineer isolated the issue to a known anomaly that Apple is already working on but has not confirmed to techs or the public.

In case anything similar comes up for your other customers, it occurs in Exchange accounts when iCloud mail is synced in Mac on the same device.

In that event, Mail apparently muddles iCloud and Exchange authentication, though how or why isn’t actually clear.

So if you have this issue, contact MAC/Apple’s 2nd Tier Support team and have them implement the fix for their program.