I got to discuss about this issue because it seems to be getting decently common and I have been involved in the resolution of this scenario. An organization has two Exchange organizations. ExchOrg1 and ExchOrg2. ExchOrg2 is connected to O365 and has AD Connect running. UserK is moved from ExchOrg1 to ExchOrg2, users in ExchOrg1 can see the free-busy. The Userk is then migrated to O365.
When UserK is migrated to O365, users in ExchOrg1 can no longer see the free busy of UserK. The problem is that there is no trust between ExchOrg1 and O365 so it does not know how to get that information from O365. There are 3 things you need to do
- Create an Exchange relationship between both organizations
- Create a relationship between ExchOrg2 and O365
- Sync the O365 Users between ExchOrg2 and ExchOrg1. Make sure that UserK’s onmicrosoft tenant address is stamped in the proxy address in the contacts created in Exchorg1. this is where MIM 2016/FIM 2010 R2 Galsync will come in.