Hi Nuno, I already told you a couple of things you could try in the IRC.
.- Check in somewhere (i.e.
http://www.senderbase.org ) if your home IP is blocked, and if so take the steps to de-list your IP and check afterwards.
.- Enable smtpd_sasl_authenticated_header = yes in main.cf, It will disclose the username you are using to authenticate, so think about the implications before enabling it in postfix.
When you have either a whitelisted IP or an authenticated mail, google should happily accept your mails, it's probably not related to ispcp
Cheers