Hello @CCMG ,
I'd advise reaching out to the Delivery team for specialized insight and troubleshooting regarding delivery issues. Calling in will also allow you to share the contact's info in a more secure manner.
Here are some general best practices for delivery. Delivery can have a lot of variables, from elements on our side, on your side, on the recipient's side, and in their email system company's side. It's also a generally good idea to set up self-authentication, since you have your own domain.
For additional info on email authentication
If you'd like to learn more about safelisting, and what it entails:
Safelisting domains in a security system
Safelisting email addresses in an email client or security system
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