The existence of this attribute tells the provisioning process that the mailbox already exists on-premises and may be migrated here later and so not to create a conflicting mailbox. When Exchange Online needs to provision a new mailbox, it will not do so where the ExchangeGUID attribute already exists. When a user is given an Exchange Online licence, it becomes the job of Exchange Online to provision a mailbox for this user. This ensures that the ExchangeGUID attribute from the on-premises mailbox is synced into Exchange Online for your tenant.
The Exchange Online directory takes a sync of information relating to Exchange from Azure Active Directory (Azure AD), which is known as forward sync. Exchange Online reads its attributes from the Exchange Online Directory Service (EXODS). Exchange Online though does not read attributes from Azure Active Directory. When Active Directory is synced to Azure Active Directory, the ExchangeGUID attribute for the on-premises user is synced to the cloud (assuming that you have not done a limited attribute sync and excluded the Exchange attributes from syncing to AAD – as syncing the attributes is required for Exchange Online hybrid). With a hybrid Exchange Online deployment, where you have Exchange Server on-premises and Exchange Online configured in the cloud, and utilising AADConnect to synchronize the directories, you should never find that a synced user object is configured as both a mailbox in Exchange Online and a mailbox on-premises.
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