I have recently started turning off smtp relaying on your main exchange server (uncheck allow all computers which successfully authenticate to relay regardless of the list above). I also read this post in the MS IMF guide:
Helping to Secure Your Gateway SMTP Virtual Servers
Dictionary attacks are brute force attacks that use common words as possible passwords to discover valid passwords for well-known accounts, such as the administrator account. Malicious users attempt dictionary attacks to gain access to computers.
To help protect your SMTP gateway servers from possible dictionary attacks, you can disable all forms of authentication on your inbound SMTP virtual servers that accept Internet mail. Because no authentication is permitted, malicious users cannot use dictionary attacks to discover passwords and authenticate to your computer to relay mail or perform other unauthorized actions--
In Exchange System Manager, expand Servers, expand <your inbound Exchange server>, expand Protocols, and then expand SMTP.
2. Right-click the inbound SMTP virtual server, and then click Properties.
3. Click the Access tab, and then click Authentication.
4. In Authentication, clear the Basic authentication and Integrated Windows Authentication check boxes.
Is this a good idea and when would you NOT want to do this?