IMAP to Microsoft 365 migration — batch or one mailbox at a time?

General Discussions
1

We’re preparing a Microsoft 365 migration for several users whose existing mailboxes are IMAP-based. The number of accounts makes doing everything manually pretty unattractive.

Microsoft’s own documentation confirms that IMAP migration can move mail folders but doesn’t handle contacts, calendars or tasks, so we’re treating those separately.

I’ve been testing an IMAP migration tool from DRS Softech because it has both Single Account and Batch/CSV modes. The batch option seems much more practical when you have a lot of mailboxes because account details can be supplied together rather than repeating the same process for every user.

Another useful part is the mailbox preview. Before migrating, you can see folders, subfolders and email counts and select what actually needs to be transferred. It also supports direct migration to Office 365, Google Workspace, or another IMAP server.

For anyone who has handled a medium-sized IMAP-to-Microsoft 365 project: would you recommend piloting a few accounts first and then using CSV batch migration for the rest? I’m particularly interested in how people verify folder counts and missing messages after the cutover.

2

For IMAP to Microsoft 365 migration, batch migration is the better choice when several mailboxes are involved. One mailbox at a time is fine for a small setup, but with more users, you end up repeating the same authentication, folder selection and verification work.

That is where WholeClear IMAP to Office 365 Converter makes more sense. You can connect the source IMAP accounts, move multiple mailboxes, choose specific folders, and use filters for things like date range or sender instead of transferring the complete mailbox every time. Emails can be moved with their attachments, technical properties, and existing folder structure.

So I’d handle it like this: test one mailbox, verify a few folders and attachments in Microsoft 365, then run the remaining accounts in batch. The current version of the tool supports Windows, macOS, and Linux.