Is it possible to detect in postfix that a sending IP is spammed by a receiving mailbox like gmail/yahoo/outlook/hotmail etc?
For example if a receiver spams my sender domain or an IP is blacklisted or sending address is spammed.
Where do I read reply headers in postfix?
No.
Reporting of an email as spam to a webmail provider is handled entirely within that providers' systems. The report is between the sending and the webmail provider.
Reporting an email as spam to the service provider of the spam origin generally goes upstream of the machine that actually sent the spam out (and if it isn't, it will be in the form of an email to an administrator's mailbox so you should look there).
Reports to a blacklist are between the reporter and the blacklist. You can monitor the blacklists (using whatever APIs they provide) to see if your IP appears on them.