Some source email systems such as Courier IMAP don't support using mailbox admin credentials to migrate mailboxes to %BRAND_OFFICE_365_SHORT%. ...

Some source email systems such as Courier IMAP don't support using mailbox admin credentials to migrate mailboxes to %BRAND_OFFICE_365_SHORT%. Instead, you can set up your source email system to use virtual shared folders. Virtual shared folders allow you to use the mailbox admin credentials to access user mailboxes on the source email system. For more information about how to configure virtual shared folders for Courier IMAP, see Shared Folders.