If the contact isn't showing as a bounce, and the email isn't ending up in their spam, then something on the receiving end is quarantining the email / preventing it from even pinging us that it's being bounced back. 

Besides safelisting your email addresses in their system, I'd recommend the following:

• Send their IT this article, which covers the domains we use that should also be safelisted.
• Work with your IT to get your account setup for self-authentication. Since you have your own domain, this becomes particularly necessary to ensure trustworthiness with more stringent email security, and thus improve delivery.

If you have additional questions regarding those processes, or need further assistance regarding your delivery, I'd advise reaching out to our Delivery team.

See also:

Email Authentication FAQ