Hello @BradM737 ,
Yes, this would be a firewall issue. If you're going to be sending emails through us using their addresses and domains, then it's going to be necessary to setup the Constant Contact account with self-authentication.
It may also help to make sure the email addresses being used for sending, as well as our domains, are safelisted in the network's security:
Safelisting domains in a security system
Safelisting email addresses in an email client or security system
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Deliverability agents.
See also:
Email Authentication FAQ
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