A client or external partner's mail server is not successful in negotiating TLS with some of our mail towers. They are able to connect successfully with other mail towers in TLS.
You are seeing the error below:
Certificate verification failed: self signed certificate in certificate chain.
A possible cause is that one of our root certificate or intermediate certs may not installed on your mail server. Please ensure that all our public root certificates are trusted by your mail servers.
To obtain our latest certificate chains please see: Certificate Authority used by the Email Security.cloud infrastructure