Why do large mailbox migrations to M365 fail?
Migrating large mailboxes to the cloud often fails because storage add-ons cannot be applied to mailboxes that are not yet present in the tenant, and tools like Purview throw generic errors during the chunk download process. These roadblocks prevent users from successfully transferring their data and lead to persistent export failures.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Temporarily using a higher-tier license with a native 100GB cap to perform the migration, then switching the license back.Did this help you?
- 2Turn off automapping via PowerShell and manually add the mailbox to download the local file.Did this help you?
- 3Run SCANPST to check for and fix database errors.Did this help you?
- 4Create a blank PST file and manually copy folders or use drag-and-drop while disconnected from the internet.Did this help you?
- 5Use third-party paid conversion or mailstore software like CodeTwo or MailStore.Did this help you?
- 6Manual clean up before migrationDid this help you?
- 7Checking with support for migration thresholdsDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Jose has a 38 GB mailbox. Tried it on Purview, I get generic errors when trying to download the largest allowable chunk size of 10GB. I can't seem to get around it and Purview is notoriously like this.”source ↗
“Export is around 33GB and it fails, same error, different email/folder.”source ↗
“Apparently Intermedia has no limits on Exchange and it resulted in some crazy counts. I don't think we can simply migrate out without cleaning up first, or can we?”source ↗
“Calendar 17,172 items / 6.82 GB and 65,167 items / 114 GB”source ↗
“Mailbox sizes over 200GB”source ↗
“I have a mailbox I want to migrate to M365 using the M365 Business Standard License. It appears the 50GB additional add-on does not apply to mailboxes that are not yet present in M365.”source ↗
“this happenend to me, iirc it happens because for some odd reason some email may be too large (might be completely wrong)”source ↗
Where this came up
People with this problem also raised
- 8How to migrate company email from Google Workspace to Microsoft 365
- 2Gotchas when migrating user files to OneDrive
- 8Migrating a child Active Directory into a parent domain
- 5Why is there a file size limit on email attachments?
- 3How to fix sync problems on large Outlook mailboxes
- 4Moving a virtual domain controller without a USN rollback