Cannot use EHLO or AUTH LOGIN outside of internal network on Exchange 2003

Our environment:

Windows Small Business Server 2003
Exchange 2003 (Note: this is the "trimmed down" version that is bundled with Windows Small Business Server 2003)

Here's my problem...  I am not being able to send email through my Exchange server using SMTP.  I can connect directly to Exchange via Outlook when inside the network, so working from within the network is not an issue.  However, when outside of our network, I try the following:

telnet mail.mydomain.com 25
I receive a string of asterisks and digits, but connect successfully...
EHLO
500 5.3.3 Unrecognized command

So then I try:

telnet mail.mydomain.com 25
I receive a string of asterisks and digits, but connect successfully...
HELO
250 mail.mydomain.com [70.33.**.**]
AUTH LOGIN
500 5.3.3 Unrecognized command

However, when I am actually on the server, and telnet to localhost port 25, I AM able to use EHLO and can authenticate successfully with AUTH LOGIN.  Please help!  Thank you!