This error is only sent back if the ReplyTo or From XML content doesn't match a verified email address on the account your setting it up from. In order to look into it, we'd need to see the XML you sent that threw the error and probably also need to get the username for the account in question. If you don't want to publish that on the forums, please direct message me the information or send an email to our webservices support team with that information (webservices@constantcontact.com).
Dave Berard
Senior Product Manager, Constant Contact
