Thank you for your quick response.
The unknown certificate authority in correct for this server. it is self-signed, and the certificate for it is trusted by the few machines that need to connect to it.
The email server has been eliminated as the source of the problem. Other clients, and yourself could connect successfully to it.
I Rolled back to 1.6.14, however the connection still failed after STARTTLS.
I noticed the connectitivity test fails to correctly identify the auth types supported by the mail server. I suspect this is because the ssl connection terminates early.
I have temporarily enabled non encrypted email submission, and Postman is able to successfully deliver mail.
Clearly there is some sort of interaction between PHP and SSL. Any assistance in diagnosing this would be appreciated, as non-encrypted email submission is not ideal.