Has anyone here handled an email migration from an older IMAP-based mail server to Microsoft 365? I am particularly interested in the practical issues that appear after the migration starts. The technical setup seems straightforward at first, but things like folder mapping, duplicate emails, attachments, and newly received messages can make the process more complicated.

One thing I noticed while researching migrate IMAP to Office 365  is that the approach can vary considerably depending on the number of mailboxes involved. For a single account, manually entering the server details and transferring the required messages may be manageable. However, when an organization has dozens or hundreds of accounts, maintaining a proper source-to-destination mapping becomes much more important.

Another area that seems worth planning in advance is filtering. Not every mailbox necessarily needs to be transferred in its entirety. Some organizations may only need emails from a particular date range, specific folders, or messages matching certain criteria. Having these controls before starting the migration can prevent unnecessary data transfer and make verification easier.

I also think incremental processing deserves attention. Users continue receiving emails while a migration is running, so a method that can identify previously transferred messages and process only newly added data could make the final migration stage much easier. Duplicate handling is equally important because repeated messages can make the destination mailbox difficult to manage.

For administrators, reporting is another useful consideration. Being able to see completed, skipped, or failed items can help identify problems before users start working from the new mailboxes.

While comparing different approaches, I also came across the DRS Softech IMAP Backup Tool. Its listed capabilities include Batch/CSV migration, mailbox preview, date and advanced email filters, duplicate removal, incremental migration, attachment options, and detailed reports. It also supports migration from services such as Gmail, Yahoo, Zoho, AOL, and Outlook.com.

I am curious how others handled similar projects. Did you perform the migration in one large batch, or did you first test a few mailboxes and then gradually move the remaining users? Also, what did you find most important for verifying that the folder structure and attachments were transferred correctly?