Mobile-originated or "MO" messages, commonly referred to as "inbound," come into the TextPower system. If the keyword to which the sending number is opted in is set up as a POST routing (i.e., TextPower sends the information contained in the message to another server) we then attempt to reach the server whose URL and credentials we were given.
When we attempt to reach that server we will either receive a "Success" indication, meaning that the connection was made between the two servers. If you see a "-6: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel." this is almost certainly due to a certificate issue.
A non-self-signed SSL certificate (in other words a certificate from an independent and recognized "certificate authority" - called a "CO") must be installed at the server that TextPower is attempting to reach or the connection will fail.
This error cannot be resolved by TextPower. It must resolved on the server side where TextPower is attempting to send the message.