Forum Discussion
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:
- Is this migration speed expected when using the IMAP migration method from Google Workspace to Exchange Online?
- Does Exchange Online enforce any throttling limits that can affect IMAP migration throughput?
- Can split routing or coexistence between Google Workspace and Microsoft 365 impact IMAP migration performance?
- Is there a recommended number of concurrent mailbox migrations or migration batch size for optimal performance?
- Are there any Google Workspace-side limitations, API restrictions, or throttling settings that should be reviewed?
- What Microsoft-recommended configurations or best practices can help improve IMAP migration speed?
- If higher migration throughput is required, does Microsoft recommend an alternative migration approach instead of IMAP?
- 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.
2 Replies
A 10 GB Google Workspace mailbox taking nearly two weeks through IMAP is unusually slow, so I would first check the migration throttling and Google Workspace-side limits rather than assuming split routing is the cause.
Exchange Online does apply throttling to IMAP migrations, and Microsoft recommends testing different migration batch sizes and concurrent connections to find a suitable level. On the Google side, bandwidth limits can also affect large mailbox migrations, particularly when there are many messages and attachments.
Split routing/coexistence mainly affects mail flow. If mail flow is already working normally, I would compare migration performance across a few test mailboxes before changing the coexistence setup.
If the standard IMAP migration remains too slow, a direct IMAP to IMAP route is another option. WholeClear IMAP to IMAP Migration connects the source and destination IMAP accounts directly using their credentials. It supports Google Workspace and Office 365, lets you select folders or apply date filters, and transfers emails with attachments while providing a migration report.
This can be useful when the requirement is to move mailbox data directly between IMAP accounts without creating intermediate mailbox files. However, it will not bypass provider-side throttling, so I would test it with a small mailbox first and compare the actual transfer speed.
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
