What I can't understand is why my router was allowing access to the mail server i.e. port forwarding port 25. I have a vague feeling that it was another half completed project that I was playing around with years ago.
Anyway, firewall is working correctly and the mail server is configured a bit better now. These are the changes I made to my postfix configuration file (main.cf):
smtpd_client_restrictions = permit_mynetworks, reject
smtpd_helo_restrictions = reject_invalid_helo_hostname,
reject_non_fqdn_helo_hostname, reject_unknown_helo_hostname
smtpd_sender_restrictions = reject_unknown_sender_domain
smtpd_recipient_restrictions = permit_mynetworks,
reject_invalid_hostname, reject_unknown_recipient_domain,
reject_unauth_destination, reject_rbl_client sbl.spamhaus.org
smtpd_data_restrictions = reject_unauth_pipelining
