When you try to migrate public folders to Exchange Online, the migration fails and you receive an error message that resembles the following:
MigrationPermanentException: Error: Public folder "/<FolderName>" couldn't be mail-enabled. The error is as follows: "No MailPublicFolder entry was found in Active Directory with OnPremisesObjectId='4a66593b-12b0-4742-9e39-1a7dccd5fc7a' or LegacyExchangeDN='/CN=Mail Public Folder/CN=Version_1_0/CN=e71f13d1-0178-42a78c4724206de84a77/CN=000000001A4473 90AA6611CD9BC800AA002FC45A0300E7243831D8E5
0446AB3A0E4E3AEBE23500000000715A0000'". This failure can happen when MailPublicFolder objects in Exchange Online are out of sync with your Exchange deployment. You may need to rerun the script Export-MailPublicFoldersForMigration.ps1 on your Exchange server, followed by the script Import-MailPublicFoldersForMigration.ps1 on Exchange Online to update MailPublicFolder objects in Active Directory.
This problem occurs if the NON_IPM_SUBTREE system folder contains mail-enabled public folders. Public folders in the NON_IPM_SUBTREE folder don't have to be mail-enabled. They may have been previously mail-enabled because they were in a Microsoft Exchange 2000 Server environment that was running in mixed mode. In this situation, Exchange tries to mail-enable all public folders.
Mail-disable all public folders that are mail-enabled in the NON_IPM_SUBTREE folder. For more information about how to mail-disable a public folder, see Mail-enable or mail-disable a public folder.
To mail-disable public folders on NON_IPM_SUBTREE, run these commands:
For public folders on Exchange Server 2010
List mail-enabled public folders from NON_IPM_SUBTREE: