Forum Discussion

Gagandeepkaur370's avatar
Gagandeepkaur370
Copper Contributor
Sep 14, 2026

IMAP Mail Migration

Hello Microsoft Community,

We are currently performing a mailbox migration from Google Workspace (Gmail) to Microsoft 365 Exchange Online using the IMAP migration method.

The customer's environment is configured for split routing/coexistence, where both Google Workspace and Microsoft 365 are actively being used during the migration process. Mail flow is functioning as expected; however, we are experiencing extremely slow migration performance.

For example, a mailbox containing approximately 10 GB of data is taking nearly two weeks to migrate, which is significantly impacting the overall project timeline.

We would appreciate guidance on the following:

  1. Is this migration speed expected when using the IMAP migration method from Google Workspace to Exchange Online?
  2. Does Exchange Online enforce any throttling limits that can affect IMAP migration throughput?
  3. Can split routing or coexistence between Google Workspace and Microsoft 365 impact IMAP migration performance?
  4. Is there a recommended number of concurrent mailbox migrations or migration batch size for optimal performance?
  5. Are there any Google Workspace-side limitations, API restrictions, or throttling settings that should be reviewed?
  6. What Microsoft-recommended configurations or best practices can help improve IMAP migration speed?
  7. If higher migration throughput is required, does Microsoft recommend an alternative migration approach instead of IMAP?
  8. If split routing/coexistence must remain active throughout the migration project, what is the recommended Microsoft-supported migration method?

We are trying to identify whether the bottleneck is on the Exchange Online side, Google Workspace side, or related to the coexistence configuration.

Any troubleshooting guidance, performance recommendations, or Microsoft documentation references would be greatly appreciated.

exchange
exchange online
office 365
outlook

1 Reply

  • Jamony's avatar
    Jamony
    MCT
    Sep 15, 2026

    A 10-GB mailbox taking two weeks is not automatically expected, but it can happen with IMAP because the migration service, source server, internet path, and batch concurrency all constrain throughput. Split routing normally affects mail flow rather than the data copy itself, so first prove the source connection from outside the network. Run several small, comparable test batches and record elapsed time, source IMAP connections, firewall limits, and Google-side quota or throttling events. In Exchange admin center, review the IMAP migration endpoint’s concurrent-migration setting; Microsoft documents 20 as the default and permits adjustment, but raise it gradually only while the source remains healthy. Also compare a test mailbox download through a normal external IMAP client. That separates source or network limits from Exchange Online. IMAP moves mail only, so if you require richer mailbox data or substantially higher throughput, assess Microsoft’s supported Google Workspace migration approach before changing coexistence design