I ran into an issue recently trying to utilize Mandrill’s web hooks on a site forcing HTTPS (duh). After sending the test webhook from the web interface, I saw the following error:

After doing a bit of internet searching, and testing different server configurations, I figured out that I did not concatentate my SSL certificates correctly.

I bought my SSL cert from Comodo, and after paying for the certs I received four files: AddTrustExternalCARoot.crt, STAR_mysite_com.crt,COMODORSAAddTrustCA.crt and COMODORSADomainValidationSecureServerCA.crt

The final solution was to concatentate them together in the following order: