Cc: email stopped working

A couple weeks back, one of my forms stopped emailing to the address in the “Cc:” under “Additional Headers”. Emails to the primary address continue to be sent to fine. Both addresses are in the same domain. I tried taking the address from the Cc: line and adding it to the To: line, comma-separated. Same result. The email in question is working – able to receive other emails and I ensured it was not caught in my spam or in quarantine at the server level.

Is there a log I can review somewhere that shows the format of the email that was sent?