Refer to Microsoft KB article 324958 for more information on how to lock down your SMTP gateway.
I would also suggest checking with http://www.ordb.org/faq/ for more information on this.
Do you have comments on this Ask the Expert question and response? Let us know.
Dig Deeper on Exchange Server setup and troubleshooting
Related Q&A from SearchExchange Staff
Get step-by-step instructions on how to designate recipients of Exchange Server non-delivery reports (NDRs) using Exchange System Manager. Continue Reading