Iterable uses the Apache Commons Email Validator. We use this validation method whenever you perform a CSV user import in the application or when you add/subscribe users via the API.
This implementation is not guaranteed to catch all possible errors in an email address.
The Apache Class Email Validator checks to make sure the address is formatted like a proper email and that it comes from a valid TLD (top level domain or the part that comes immediately after the dot symbol in an email address).
For more information, you can check the code for Apache Commons Email Validator here.