i saw several times in some forums, that many have the problem, that their mails are marked (false!) as spam, because their computer have dynamic ip's. to avoid, that your customers have the same problem, you have to add this line to
smtpd_sasl_authenticated_header = yes
if you add this line (and restart postfix), that postfix adds to the header, that the dynamic IP, the PC uses is "ok". and the false SPAM-Problem "is gone".
maybe you can add this to your perfect-server-setup howto ;-)