I'm afraid that we need a new smtp port for SOGo in this case, but with default iRedMail settings, it works fine.
*) Add new lines in /etc/postfix/master.cf:
127.0.0.1:2525 inet n - n - - smtpd
-o syslog_name=postfix/plain-smtp-2525
-o smtpd_sasl_auth_enable=yes
-o smtpd_tls_security_level=may
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
-o content_filter=smtp-amavis:[127.0.0.1]:10026*) In /etc/sogo/sogo.conf, update "SOGoSMTPServer" setting:
SOGoSMTPServer = 127.0.0.1:2525;*) Restart both postfix and sogo services.