IMAP to Microsoft 365 migration keeps failing on large mailboxes what worked for you?

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I’ve been struggling with a mailbox migration project and wanted to see if others experienced the same thing.

The environment contains several large IMAP mailboxes, some exceeding 8–10 GB. Using Microsoft's standard migration process seemed fine initially, but larger mailboxes frequently stalled or completed with missing folders. Re-running migration batches became a repetitive task, and users were concerned about missing messages.

Since I needed a more reliable solution to Migrate IMAP to Office 365, I explored specialized migration tools. One option that performed well was the DRS Softech IMAP Backup Tool. The software allowed me to connect directly to the source IMAP accounts and migrate data into Office 365 while maintaining mailbox hierarchy. I also appreciated the filtering options because they helped reduce unnecessary data transfer during testing.

The biggest improvement was consistency. Instead of constantly monitoring migration batches and troubleshooting sync issues, the migration completed with minimal intervention. For organizations handling multiple mailboxes, this can make a significant difference.

If you're currently migrating IMAP to Office 365 and experiencing failures with larger mailboxes, it may be worth looking beyond Microsoft's native tools. Sometimes the simplest solution is using a migration utility specifically designed to handle complex mailbox transfers efficiently.
 

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