newbee help on freebsd email server setup

[Dennis M. Yocum]

hello
still a newbee too

i would like to ask a few questions in setting up my mailserver

environment is simple.....intel based solely running freebsd 4.7 right now wnd sendmail 8.12.6 and qpopper 4.0.4

all are running without error.....i think

the host command is replying correctly
my remote dns service a and mx records are fine
ping to the name resolves correctly
host-names file modified with names
inetd running modules without error messages

mailserver sees internet ok

here is the problem i cant track down.....when i send mail to an account set up on mailserver (thru adduser) 
and using an remote source email server.....when i use a pc through windows 2000 running outlook express i cannot retrieve the mail...it comes back with error message 550 host unknown....

can anyone steer me in the right direction.....checked freebsd handbook....freebsd in 24 hours....and usual sites faq...

thanks
den