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]
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?
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
You were correct regarding the linked post you shared, in that the error message 'Invalid email address' is typically caused by our compliance system detecting multiple sign ups to several Constant Contact accounts from the email address that caused the error.
This issue is not specific to calls made via the API, and can happen with any sign-up form, even those provided directly by Constant Contact. The compliance check is in place to prevent bad or flagged addresses from being added to your account, since if you're frequently sending to bad email addresses, it can start to affect your email's deliverability.
I can confirm that all of the addresses that you listed have been detected as having signed up to multiple accounts in a short time causing the system to block sign ups from these addresses. However, if you were testing with these addresses (hence multiple attempts), or if you know the contact that is trying to sign up and that it is a valid address, you should still be able to add them to your contacts manually.
If you have further questions about these error messages please feel free to reach out to our Account Review department and they would be happy to answer any concerns you may have. Your case number to reference is 30243227.
Contact an Account Review and Delivery Specialist
Please have a look and let us know if you have any other questions!