Throttling limits during Office 365 tenant-to-tenant mailbox migration via IMAP

Rohit Raj Kumar 100 Reputation points
2026-07-02T07:59:05.2733333+00:00

I'm working through an Office 365 tenant-to-tenant migration where mailboxes are being moved via IMAP rather than native cross-tenant mailbox migration, since the source tenant setup doesn't support the native admin-driven method in this case.

During larger batches, I'm seeing intermittent throttling and slower-than-expected transfer rates, even though the EWS/IMAP throttling policies documented by Microsoft suggest the limits should comfortably handle the batch sizes I'm using.

Questions:

  1. Are IMAP-based migrations into Microsoft 365 subject to different (stricter) throttling thresholds than EWS-based migrations, or should they behave the same under Exchange Online's throttling policy?
  2. Is there a recommended batch size or concurrency limit specifically for IMAP migration into M365 that isn't fully covered in the general throttling documentation?
  3. Does enabling any specific mailbox or tenant setting (e.g., migration endpoint configuration) help reduce throttling during IMAP-based moves, similar to how migration endpoints are tuned for native cross-tenant migrations?

Any pointers to official throttling documentation specific to IMAP migration scenarios, or real-world experience with batch sizing, would help.

Exchange | Exchange Server | Management
Exchange | Exchange Server | Management

The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.

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Answer accepted by question author
JimmySalian-2011 45,976 Reputation points Volunteer Moderator
2026-07-02T08:21:43.62+00:00

Hi Rohit,

There are some limitation and they are listed in the MS documentation, I believe you might be hitting the connectinsand mailbox items limit.

  • Microsoft strongly recommends disabling all MRM and archival policies before attempting any data migration to mailboxes.
  • You can only migrate items in a user's inbox or other mail folders. This type of migration doesn't migrate contacts, calendar items, or tasks.
  • You can migrate a maximum of 500,000 items from a user's mailbox (emails are migrated from newest to oldest).
  • The biggest email you can migrate is 35 MB.
  • If you limited the connections to your source email system, it's a good idea to increase them to improve migration performance. Common connection limits include client/server total connections, per-user connections, and IP address connections on either the server or the firewall.

https://learn.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/migrating-imap-mailboxes

I will suggest trying PowerShell route to migrate - https://learn.microsoft.com/en-us/microsoft-365/enterprise/use-powershell-to-perform-an-imap-migration-to-microsoft-365?view=o365-worldwide

Hope this helps.

JS

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