it's a good chance while i'm not in Singapore to verify if the tcpserver works correctly.
the Squirrelmail client works fine w/o setting anything. Thunderbird mail client could not access the SMTP server.
Then executive the following:
amend tcp.smtp to add the WAN address of the gateway
cd /etc; tcprules /etc/tcp.smtp.cdb /etc/tcp.smtp.tmp < /etc/tcp.smtp
reload smtp cdb by executing 'qmailctl cdb'
Thunderbird mail client worked properly.
See Qmail Setting Up Servers.