Migrating IMAP Mailboxes to Exchange Online?

A few observations that are NOT mentioned n the documentation: https://learn.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/migrate-other-types-of-imap-mailboxes

1. The destination mailbox is first in the file (it does actually state this later on in the how to if you RTFM).
2. Easiest to just do one mailbox per migration batch. (When there is an error, is easiest to redo individuals and remember where you got up to).
3. You CANNOT have two batches running (or even scheduled to run), with the same mailbox destination.

Why would you even have this situation you may ask..

Let's say there are some historic mailboxes at the old host that are ging to be abandoned.. why not migrate those all into one [free] Shared mailbox, just in case they are needed?

You would think you could just specify a csv like this:

history@newhost.com, furst@oldimaphost.com,password124

history@newhost.com, shecond@oldimaphost.com,password124

history@newhost.com, thurd@oldimaphost.com,password124

No such luck, each one needs to be run on its own, completed and then deleted before the next one can be started!

One further undocumented issue - can a migration be done to a Shared mailbox (so not requiring a license)?

Not sure, because I added it as a licensed user first, before I realised the issue above...

I might remove the user's license now the job done, and leave it as a shared mailbox = which generously currently allows a 50GB mailbox on the house!

Also. after each migration, it may be to drag the content into a new subfolder per person, so as not to have have a hug mixed up Inbox.

Which would be ideal if the webmail page allowed this. Drag an drop worked well for folders, but select all items in the Inbox, and they simply would not drag or (right-click Move) to another folder.

Oh well. No-one is ever going to look in it anyway. And Search is your friend.

Oh, and don't forget to complete the batches BEFORE you change the MX records.. although it does still work if you don't (ask me how I know)

Oh - here is the AI take on things:

To migrate multiple IMAP mailboxes into a single shared mailbox in Exchange Online, you generally need to migrate each source account into a temporary licensed user mailbox first, then merge or move the data into the target shared mailbox. Microsoft’s native IMAP migration tool is designed for 1-to-1 mailbox transfers; it does not natively "merge" multiple source accounts into one destination in a single step. 

Recommended Migration Workflow

1. Prepare the Target Shared Mailbox: Create the shared mailbox in the Exchange Admin Center (EAC) and ensure it has a large enough storage limit (default is 50 GB for unlicenced shared mailboxes).
2. Create Temporary User Mailboxes: For each IMAP account you want to migrate, create a corresponding user in Microsoft 365 and assign a temporary license.
3. Run the IMAP Migration