You can migrate the contents of user mailboxes from your source email system to Microsoft 365 or Office 365. Use the Internet Message Access Protocol (IMAP) to migrate email when:
Things to consider
Here are a few limitations to be aware of:
You can only migrate items in a user's inbox or other mail folders. This type of migration doesn't migrate contacts, calendar items, or tasks.
You can migrate a maximum of 500,000 items from a user's mailbox (emails are migrated from newest to oldest).
The biggest email you can migrate is 35 MB.
If you limited the connections to your source email system, it's a good idea to increase them to improve migration performance. Common connection limits include client/server total connections, per-user connections, and IP address connections on either the server or the firewall.
Impact of migration to users
To migrate email, you need access to the user mailboxes in your source email system. If you know the user passwords or can access their mailboxes by using administrator credentials, there won't be any impact to users until you shut down your source email system.
If you can't access user mailboxes, you'll have to reset the passwords. This lets you access the user mailboxes by using a new password that you know. If users don't know the new passwords, they won't be able to get to their old mailboxes during or after the email migration. You can distribute the new passwords after the migration if you want users to get to their old mailboxes
For more see the Microsoft article :
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