we are in hybrid setup how to change the domain name and change users email address

exchange is there on cloud in m365 (exchange online)

how to change active directory domain name?

Hi Doddabasappa K

Based on my research, a common approach to minimize user impact in a hybrid Exchange environment is to make all mail recipient changes on-premises first and allow Microsoft Entra Connect to synchronize those changes to Microsoft 365. Avoid modifying synchronized users directly in Exchange Online, as those changes are typically overwritten during the next synchronization cycle. 

While the exact process depends on your environment, it generally involves the following steps: 

• Add and verify the new email domain in Microsoft 365, and add it as an Accepted Domain in your on-premises Exchange organization. 
• Update users' email addresses on-premises by changing the primary SMTP address to the new email domain while keeping the old SMTP address as a secondary alias during the transition. This allows mail sent to either address to continue reaching the same mailbox. 
• If you also intend to change users' sign-in names (UPNs), update the UPNs on-premises after adding the new UPN suffix in Active Directory, then allow those changes to synchronize. 
• Verify that mail flow, Outlook connectivity, and user sign-in are working as expected before removing the old email domain. 
• Only remove the old email domain after confirming that it is no longer referenced by any users, groups, shared mailboxes, contacts, or other mail-enabled objects. 

I hope this information is helpful. 

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