Your comment about only the receiving server certificate being used is interesting.  We're following the Microsoft article linked below for setting up a certificate-based connector.  To answer your question, I do see to connections in the log.  The message is absolutely using TLS 1.2 (verified in message headers and from Exchange Online message trace).  The Email Sender App shows the third party certificate is being used when relaying to the Xeams server.  

Configure a certificate-based connector to relay email messages through Microsoft 365 - Exchange | Microsoft Learn

Thank you.