If you are unable to connect to an iCloud account (@firstname.lastname@example.org for example), you will most likely need to create an app-specific password and then use that in the password field in Aid4Mail. You may also need to try using just the username in the "User name" field instead of including the full email address (with domain).
Directions on how to generate an app-specific password are found here: https://support.apple.com/en-us/HT204397
Two-Factor Authentication has to be turned on in order to generate an app-specific password. Once you finish the collection, you can turn it back off.