Moving an organization from an old mail server to Microsoft 365 can be a bigger task than expected, especially when users have years of email stored in their IMAP accounts. The goal isn't only to transfer messages but also to make sure users can continue working with their familiar folders and important correspondence after the change.

When you migrate IMAP emails to Office 365 , I think a phased approach makes more sense than attempting everything at once. Administrators can begin with a small group of mailboxes, check the connection, review the transferred folders, and then gradually move the remaining accounts. This also gives users an opportunity to report anything that looks different.

I would also create an IMAP email backup before beginning the main migration. Keeping a separate copy of the source mailbox means important messages remain available if something needs to be verified later. It can also be useful when an organization plans to retire the old email server once the migration is complete.

Another consideration is mailbox cleanup. Some accounts may contain years of unnecessary newsletters, duplicate messages, or outdated attachments. Reviewing this data beforehand can reduce the volume that needs to be processed, although critical business correspondence should obviously be preserved.

For organizations managing many accounts, the SysInfo IMAP Backup Tool is one solution I found while researching migration options. Its features include batch processing, CSV support, folder mapping, duplicate filtering, date-based filtering, incremental migration, and multiple export formats.

I think communication is also an underrated part of migration planning. Users should know when their mailbox is being moved and what they should check afterward. Administrators can then verify important folders, recent messages, attachments, and mailbox accessibility before closing the old environment.

For those who have migrated IMAP accounts to Microsoft 365, did you move users in small groups or complete the migration in one operation? Which approach caused fewer issues for your organization?