IMAP migration fails to add user

I am attempting to migrate from my old IMAP account to using Zoho Mail. I have added the server settings in a new migration and that seems to have worked.

When I add a user to the migration it fails and says 'User not exist'. What does that mean and how do I diagnose the error further? I am adding the user name and password exactly as they are in my IMAP client and then putting my Zoho email address in the third box, hope this is correct.