On your exchange server try doing the following commands:
c:\> nslookup
> set type=mx
> set nosearch
> yahoo.com
At this point you should get about 10 MX records for yahoo, like this:
[blue]Non-authoritative answer:
yahoo.com MX preference = 1, mail exchanger = mx2.mail.yahoo.com
yahoo.com MX preference = 1, mail exchanger = mx3.mail.yahoo.com
yahoo.com MX preference = 5, mail exchanger = mx4.mail.yahoo.com
yahoo.com MX preference = 1, mail exchanger = mx1.mail.yahoo.com
<etc...>
mx1.mail.yahoo.com internet address = 64.157.4.78
<etc...>
[/blue]
If you don't get that, then you need to check what DNS's that your exchange server is using.
If you do get that, then you should try:
c:\> telnet mx1.mail.yahoo.com 25
You should get the mail server header from the yahoo server. If you don't, then check your firewall to see if it allows outbound SMTP.
Lastly, some mailhosts on the internet require that your outside IP address have a reverse DNS record. To check this, do this command:
c:\> nslookup <your outside IP>
If you get a response that it doesn't exist, then contact your ISP.
PSC
Governments and corporations need people like you and me. We are samurai. The keyboard cowboys. And all those other people out there who have no idea what's going on are the cattle. Mooo! --Mr. The Plague, from the movie "Hackers