Unsupported Providers - IMAP Option
If the email address you are trying to migrate from is not supported, you can use the IMAP option. For this option, you will be required to enter the source provider’s IMAP address.
Whether you selected your host’s IMAP address through the drop-down menu or entered it yourself, the address has to start with either “imaps://” or “imap://” where the extra “s” is a designation of whether secure connection is enabled on your source account. If you are unsure, try both to see what works with your provider’s IMAP address.
And Step 3 is simply the email address you want to be contacted at once your migration has begun, as well as when your migration has completed.