SSL issue between Mendelson AS2 and our SAP PI 7.31

Dear ladies and gentlemen,

we try to install the AS Mendelson Server. The installation was succesfully done.

We create the partners and imported the Key's / Certificate.

But if we try to send a message to our SAP PI we get always back follow error message:

mendelson_opensource_AS2-1455790742094-2@SOURCE_AS2_ID to TARGET_AS2ID: MessageHTTPUploader.performUpload: [SSLHandshakeException]: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

I imported our own certificates. I let the settings in the secure tab as it is.

Thanks for your help