We are trying to make an outbound call to an endpoint on our webserver (https://atlasnetwork.org/) however we are getting an error "javax.net.ssl.SSLPeerUnverifiedException: peer not authenticated" in Salesforce outbound messaging. I've done research, and can't find the cause of the problem. Our SSL certs fingerprint matches to one on record in the Salesforce keystore and the command "openssl s_client -connect www.atlasnetwork.org:443 -showcerts -CApath /etc/ssl/certs" shows no issues with the configuration. We are running apache 2.4. The endpoint/API works over HTTP without issue.

In our Outbound Messaging Delivery Status we are receiving an error message "javax.net.ssl.SSLPeerUnverifiedException: peer not authenticated"

But, we have a valid cert as distinguished here:
https://www.sslshopper.com/ssl-checker.html#hostname=admin.clearcareonline.com
This shows that we have a valid cert chain.
https://developer.salesforce.com/page/Outbound_Messaging_SSL_CA_Certificates
starfieldclass2ca is listed and is in the chain
openssl s_client -connect admin.clearcareonline.com:443 |egrep -i starfield
s:/C=US/O=Starfield Technologies, Inc./OU=Starfield Class 2 Certification Authority

And, both the certs on Salesforce and our Site have a matching SHA1.

What needs to happen to ensure that Salesforce accepts our cert?