Hi @S.zw,
Before you delete the migration batch, the source email system is synchronized with Microsoft 365 or Office 365.Therefore, you don't have to worry about losing messages after MX points to M365.
For more information, please refer to steps 5 and 6 in this link:
Migrate other types of IMAP mailboxes to Microsoft 365 or Office 365 | Microsoft Learn
Hope it helps you!
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