When pointing the SMTP server to the Exchange 2010 mail server you get the following error:

Error Message: Can't send command to SMTP host

Exception: javax.mail.MessagingException: Can't send command to SMTP host; nested exception is: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target 

When pointing the SMTP server to the Exchange 2010 mail server you get the following error:

Error Message: Can't send command to SMTP host

Exception: javax.mail.MessagingException: Can't send command to SMTP host; nested exception is: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target 

This error happens when using the exchange server to relay mail.

Please follow the link below to set server up to relay through exchange.

http://www.petenetlive.com/KB/Article/0000542.htm