Thank you for bringing this to our attention! This is something we would like to collect more details on. For instance how long has the customer been seeing this issue? Is this email landing in your spam folder or in your inbox? We have received reports of seeing warning messages like this in Gmail. Are you only seeing this in Gmail, or is this happening in any other email client? At this time these warnings appear to only happen in Google hosted inboxes. I would suggest clicking "Looks safe" if you have not yet done so. Does clicking "Looks safe" work? Do any of the following messages have the same warning after clicking "Looks safe"? Please email us these details along with a screen shot of your email's full email header (from email, subject line, etc.) to social_support(at)constantcontact(dot)com.
In the meantime I have tracked this experience in your account and added you to our queue to be notified when this is resolved.