API Call with "Email address is invalid" Return

The API call to the endpoint https://api.cc.email/v3/contacts/sign_up_form returns "Email address is invalid" for many calls. Here are some example email addresses which look like valid but all been rejected as "invalid".

[example addresses redacted by Constant Contact for privacy purposes]

I found this article
https://community.constantcontact.com/t5/Developer-Support-ask-questions/v3-api-returns-Email-address-is-invalid-for-no-apparent-reason/m-p/370114#M12556

It looks like if you see emails having signed up to multiple constant contact accounts in a short time, you will block sign ups from these addresses.

In our application, people can sign up multiple mailing list, but they all belong to the same constant contact account.

Can explain what caused these emails been rejected?