I'm really sorry to hear that your Gmail contacts are seeing this warning from your emails, I can definitely understand how important it is to get rid of this warning. At this moment we are tracking those who have customers receiving this warning message from Gmail. Please send us an email at social_support(at)constantcontact(dot)com along with a reference to this post with the following information:
This information will help us further troubleshoot the issue. Please feel free to reach back out to us with any additional questions you may have about this, we're always happy to help!
I sounds like Constant Contact support is dodging the issue because this thread is a year old, and it wouldn't take that long to get with google and figure this out. According to google, the problem is that Constant Contact inserts "via" in the sender address info.
"your_email_address" via auth.ccsend.com
Google says that if you send messages using a bulk mailing vendor or a third party affiliates, check out Gmail's Bulk Senders Guidelines, including: Publish an SPF record that includes the IPs of the vendor or affiliates which send your messages.
Sign your messages with a DKIM signature that is associated with your domain.
Make sure the domain in the "From:" address matches the domain you're using to authenticate your emails.
Can we please get some help with how to do this from Constant Contact support?
This line of text you have pointed it out is added due to email authentication. There is also more than one way you can accomplish this authentication, ways that do not include the via auth.ccsend.com being added into your header. This Help Center article goes over what is involved in this authentication process. However the experience mentioned in this thread is mostly happening when the from email is the same as the to email.