Can receive, but not send mail on mail server

Hi all,

My mail server is running Windows 2k, and ArgoSoft MAil Server Pro
1.8.8.1 It is behind a router, so I forwarded ports 25, 110 and 8180 to
it for pop3, smtp and webmail. The DNS servers detected by the server
were 194.168.4.100 and 194.168.8.100

Tonight, I asked my friend to send me a mail as a test...seconds later,
"buh-doop" it arrived in my inbox. I then replied to him, and from
inside Outlook 2003, it appeared to have sent. But he said over a
period of 20-30 minutes that he did not get it.

Due to a Remote Desktop problem at the moment, I can't copy the log, but
when I looked at it I saw:

"Trying the server pamxl.hotmail.co.uk 64.4.45.230"
"SMTP error relaying the message to pamxl.hotmail.co.uk 64.4.45.230"
"SMTP server error: Will retry in 0:10 hours"

I'm stumped with this. Can anyone help me?

Thanks,

Neil