Cannot get email outside local domain

There are any number of things to check, most of them on the Exchange server, although network problems or misconfigurations between you and your ISP are also possible. What changes have been made that might be in the same time frame as the appearance of the problem, to the mail server or the network? Is this a new mail server, or has it worked properly previously and then stopped working? Has your ISP made any changes that might have had an impact? Is it just mail from certain domains suchas hotmail and yahoo, or is it any external mail at all?

First thing I'd check for if you can't identify a likely external cause would be a problem with the way Exchange is configured to accept connections or messages. I'm not intimately familiar with Exchange 2000, but in 5.5 there are a number of things that could account for your problem:

"Accept messages from" set to a list instead of all. . .

"Accept connections from" set to only hosts using encryption or authentication. . .

Acouple of "Client can only submit" settings. . .

Could be set not to reroute incoming SMTP mail. . .

Could even be set to allow outgoing transfers only. . .

And there are probably others I'm missing. Hope this a least points you in the right direction.

1. Use nslookup utility to verify your domain MX record.
2. Check your firewall to allow incoming SMTP traffic.