I can see you were also able to speak with our deliverability agents directly on this experience. Although that team was able to assist, we wanted to make sure our response was shared here in the Community as well. Our support was able to determine your domain is currently employing a DMARC policy that is causing your mail sent through a third-party sender (like Constant Contact) to go to spam. This team suggested implementing a DKIM key. This tool is an email authentication technique that allows the receiver to check that an email was indeed sent and authorized by the owner of that domain. Though we' re unable to assist with the implementation of the DKIM key, our Account Review department is available by phone i f you have any additional questions about the information we've provided.
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