How do I show a certificate's detail in OS X Mail (Lion)
This is not in Mail, I know of no way to do that, but this does not require any new software.  In Terminal:

echo ^d|openssl s_client -connect host:port 2&gt;/dev/null |  sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' |  openssl x509 -noout -subject -dates