I am currently experiencing a problem where people sending mail external to the company I work for are receiving 5.2.1 NDRs on messages sent to some Public Folders and Distribution Lists. It is seemingly random the ones it has chosen to do this for. If I send mail to the Public Folders and Distribution Lists internally (ie. not from the internet) all of them go through. I have checked over my settings many times and cannot see the problem. I have assigned a 500 to this question since I need this resolved as soon as possible. If you need any further information I will be glad to provide it.