I am sorry to hear that this is happening. Can you tell me how you are choosing to add contacts: type in one contact, type in multiple or paste contacts? Also, can you tell me what browser you are using when this happens? If possible, a screenshot of the error message that you get would be helpful as well.
I am having the same problem - entering e mail addresses manually, these are ".co.za" addresses from the Domain of the Company we are serving, and they are correct. Why are they showing as "Invalid address"?
I'm sorry you're experiencing this too! It may be related to your web browser. You'll want to make sure to use a compatible web browser and that it is up-to-date. In addition, please try adding those email addresses in Safe Mode/Incognito/Private Browsing. If you are able to successfully add the contacts without the error message it's most likely a third party add-on that needs disabled from your web browser. You can also try uploading the contacts from a file. If you're still receiving the error message, please send an email to social_support(at)constantcontact(dot)com along with your username, the list of ".co.za" email addresses, the name of the web browser you're using, and a reference to this post so we can further investigate for you. Thanks!