During my day to day work as a part of support organization, I work with and help troubleshoot Hybrid Configuration Wizard (HCW) failures. One of the more common causes of HCW failures is the Federat...
Updated May 05, 2025
Version 5.0
Blog Post
10 MIN READ
How to address Federation Trust issues in Hybrid Configuration Wizard (HCW)
Mirela_Buru
Microsoft
MicrosoftHCW doesn't change anything in the user attributes. It is the directory synchronization process (AADConnect) that syncs attributes from on-premises to EXO and some attributes are written back to on-premises).
In a regular Hybrid deployment, you have mailboxes on-premises, you bring AADConnect and synchronize the users to cloud. These users will be mail users in cloud with ExchangeGuid (on-premises mailbox identifier) synced from on-premises. The ExchangeGuid tells EXO that the user has a mailbox based on premises and even if you assign an EXO license to these users, we won't create another mailbox in cloud. But if you don't have the ExchangeGuid on the cloud user then we wouldn't know that the cloud user has a mailbox on-premises so we will give the user a cloud mailbox when assigning an Exchange Online license.
Here are some references on two mailboxes scenario:
https://techcommunity.microsoft.com/t5/exchange-team-blog/permanently-clear-previous-mailbox-info/ba-p/607619