both successful and failure message is hitting same URL is same.
URL : https://XXX.salesforce.com/services/Soap/c/28.0/00Db0000000Jful
Below is the Audit log of failed message.
this is HTTPS connection. required certificate is already loaded in TrustedCA. This interface working fine since 2015.
Below is the screen shot of certificate chain. Verisign Root CA is already exist in the nwa, but I am not finding other two - Symantec and Salesforce.com
reg, avinash