Free/Busy information for the user immediately fails and does not show any information in DominoFreeBusyService.exe.log. Only when you when you enable troubleshooting logging in Outlook do you see the error below.
Unable to send cross-forest request for mailbox <User Name>SMTP:UserName@Domain.com because of invalid configuration., inner exception: Configuration information for forest/domain Domain.com could not be found in Active Directory.
The account had a three letter HEX code at the end of the LegacyExchangeDN value (which is only for mailbox enabled users)
In Exchange disable the user account then create a contact (Run the Directory Connector)
It might be possible to just remove the three characters from the LegacyExchangeDN, but this has not been tested as of time of writing the solution.