IMAP to Office 365 Migration: What Can Go Wrong?

I’ve been researching IMAP migrations because our organization has several older mailboxes that need to be moved to Microsoft 365. At first, I assumed it would mainly involve entering the source and destination details, but there are quite a few things that can affect the outcome.

One important step before you migrate IMAP to Office 365 is understanding exactly what is stored in the source mailbox. Emails, folders, attachments, timestamps, and standard message flags can generally be part of the transfer, but other mailbox information may require separate planning. Knowing this beforehand can prevent surprises after the migration.

I’m also wondering how many administrators create an IMAP email backup before beginning the process. Personally, I think keeping a separate copy of important mailboxes is worthwhile. If something goes wrong during the transfer or a message needs to be checked against the original, having a backup makes the verification process much easier.

Another challenge is dealing with multiple accounts. Manually configuring and monitoring every mailbox can quickly become repetitive, especially when the organization has a large number of users. While researching possible approaches, I came across the SysInfo IMAP Backup Tool, which includes batch processing, CSV account handling, folder mapping, duplicate filtering, and incremental migration features.

I would still recommend treating the migration as a controlled project rather than simply starting the transfer. A pilot migration, documented account list, source backup, and post-migration message count check could make the entire process easier to manage.

I’m interested in hearing from people who have handled this in a real business environment. Did you encounter problems with authentication, folder mapping, duplicate emails, or missing attachments? And did you perform a complete backup before beginning the migration?