Hi @Sandor,
Sorry not sure I understand your requirement.
Would you please clarify:
- Are you asking about migrating the empty Exchange on-premises server to Exchange online? After that you would like to decommission the on-premises server.
- Or are you asking about migrating the mailboxes from third-party mail server to Exchange online?
If it is point 1, to me there is no need and also not possible to "migrate" an on-premise server to Exchange online.
Since currently users are using other email service, if you have no plan to use Exchange (on-premises or online or hybrid) as email service in the future, you can consider decommissioning the on-premises server.
If it is point 2, you can consider using IMAP migration if the email service provider supports it.
Please refer to this link for more information:
What you need to know about migrating your IMAP mailboxes to Microsoft 365 or Office 365
If I misunderstand your question, please feel free to point out.
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