Where to find the error messages of Postfix

https://serverfault.com/questions/747244/postfix-cant-send-mails-to-google-server-from-google-server

Have you already stumbled upon this comprehensive Postfix Debugging Howto? There's the following notice concerning logging:

Postfix logs all failed and successful deliveries to a logfile. The file is usually called /var/log/maillog or /var/log/mail; the exact pathname is defined in the /etc/syslog.conf file.

(syslog.conf specifies where the mail-facility logs get written to, it's rather self-explaining when you look at it.)