There appears to be a bug in Outlook that allows users to add internal accounts to the Safe Senders list. Users are adding their colleagues to these lists creates issues with automation in PowerShell.
We need to add several senders to all user accounts and it fails for the users who added the local domain(s).
My local domain is ACMEcorp.com and Outlook allows users to add email address removed for privacy reasons to their safe senders list. In PowerShell that add would be rejected. The problem occurs when you want to add a legitimate item to that user. The add fails due to a validation error on the existing list contents.
You can't remove the invalid entries with PowerShell either, the only fix is to have the end user remove them.