This means that the email address associated with the order in question is not a valid email. It's possible that the customer provided a bad email when they made the order, or something happened in the export from your other platform and the email address may have been corrupted. Shopify validates any email address you map to the email field so if the email address is not valid, the order will fail to import.
There are a couple of ways you can fix this error:
You can provide a fake email address with the orders that fail. You won't be able to communicate with this customer, and it won't populate the customer's order history, but if you need the order for your own records then this is likely the best option. Just remember that the domain has to be valid if you use this option.
Another solution would be to download the orders that have this error and then edit the preset mapping you used and unselect the email field, then re-import the failed orders.
Lastly, you could simply remove the orders in question from your order history. This isn't the best option since these orders will be missing, but if you've tried the other options then this is likely what you will have to do.
Feel free to reach out to support@orderimp.com if you need additional help.