Mailbox Migration Error

Royal D Costa 241 Reputation points

Hi All

i have a environment in which there are 2 forests AD and exchange servers, all users from both Exchanges will be migrated to Single tenant. both Exchange servers all together have more than 250 domains. right now i am facing following Challenges in creating/migrating users/mailboxes.

**Conditions - All users must be in On premises AD and mailboxes in Exchange Online **

Problem 1 - every domain has info@keyman .com email ID s, so all together we have 200 Info Id s alone, as a rule of thumb there cannot be same UPN in the Azure AD. so migration doesn't happen.

Problem 2 - for some email Id s i deleted On prem AD account and mailbox and created new new AD account on prem and allowed it to sync. after the synchronization i noticed that new user is created as info3453@keyman instead of info@keyman .com .

can someone help me to understand how exactly this works and how to migrate all mailboxes smoothly under this condition.

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Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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  1. KyleXu-MSFT 26,196 Reputation points

    @Royal D Costa

    First, one AAD could sync multiple forests: Topologies for Azure AD Connect

    If you want to keep those mailboxes using original email address(such,,, you need to add those domain name into Office 365. Otherwise, those mailboxes will use Exchange online default domain name(@keyman

    So, it will not an issue that each domain has a info mailbox, if they using different domain name.

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