SMTP issue

[Luis Rodríguez <lmrc iss com mx>]

i have a linux mail server working with sendmail, the server is behind a 
SpeedStream 5200-Series
i use port forwarding to redirect to the LAN IP of the server...

i use a valid domain name, suppose mydomain.com
i can receive messages, and send, but with some servers (the most) i 
receive an alert on the netscape mail client:
"An error occurred while sending mail. The mail server responded: 5.7.1 
<lmrc iss com mx>...
Relaying denied. IP name possibly forged [x.x.x.x].
Please check the message recipients and try again.

where x.x.x.x is the ip of the computer with the netscape mail client...

i configure the netscape mail client:
POP mail server: mydomain.com
SMTP server: mydomain.com

if i use another SMTP server like iss.com.mx then i can send the 
messages without problem...

i think this is because the server iss.com.mx detect that the message 
not came form mydomain.com, detect may the IP of the modem and then 
block the message like block spam mail or mail that not have a valid 
domain name...

can anyone help about this issue or any comment if someone have a 
related issue

thanks