IMAP Mail Migration

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.