Query on mail users in hybrid deployment

Hi @Surya kumar ,

As per the Microsoft documentation, when you create a office 365 mailbox in exchange online in hybrid deployment, then a mail user will be created at on-premises. Just trying to understand why this was happening.

The "office 365 mailbox" here means the New Office 365 Mailbox which consists of a mail user that exists in Active Directory on-premises and an associated cloud mailbox that exists in Exchange Online. This is different from the "User mailbox".

As per your concern about the "mail user", the purpose of the MailUser object in this scenario is to make is possible to manage the Exchange attributes for these users. See the blog below for more information about this:
On provisioning mailboxes in Exchange Online when in Hybrid

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