When I try to upload the csv file with bulk mail contacts for exchange organization HC10 throws the error: White spaces in the email address, even though there aren't any white spaces in the external email address. I have double checked the csv file for white spaces in the email section and there isn't any white space.

I exactly had the same issue which was resolved after providing the internal email address value in the csv file. If you still face issue, you can contact the support team as they are good at providing rapid fixes for such issues.