Upon checking the mail logs, I find this
Jul 12 07:39:35 ns2 postfix/smtp[31739]: certificate verification failed for gmail-smtp-in.l.google.com[173.194.68.26]:25: untrusted issuer /C=US/O=Equifax/OU=Equifax Secure Certificate Authority
Jul 12 07:39:36 ns2 postfix/smtp[31739]...