Question with WebMail + Mail Server

Dear All,

There is Firewall + WebMail + Mail Server in a computer machine ( Linux
Server ) :

Firewall is using iptables, protocol of WebMail ( IMP 3.2.3 ,
http://horde.org/imp ) is using imap/notls and port is using 143, and
the Mail Server is using imap ( come with Fedora Core 1 Linux ).

iptables ruls :

iptables -P INPUT DROP
iptables -A INPUT -i eth0 -p tcp --dport 143 -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --dport 80 -j ACCEPT

But I have the following the problem :
http://marc.free.net.ph/message/20040318.053223.3f53fbd8.html

So, who can help me to fix this kind of problem ?

Many thank for your help too much !

Edward.