Email is Invalid

I have set up all the settings correctly, including the advanced id set to “email”. I have it configured to connect to my Constant Contact account and selected the list. However, any email address I enter in to be added to the mailing list it comes back with Email Address Invalid. I’m entering email addresses that are not already in my list so I’m not sure why it won’t add any of them. Any suggestions? I’ve searched and searched and followed all the directions but no luck