I have set our Exchange 2010 SP3 to prevent external users to send to our internal Distribution List. In fact, the default is to Require that all senders are authenticated under Message Delivery Restrictions. However, this isn't working at the moment. I also have tried to create Transport Rules with the condition for messages outside the organization destined to members of a certain DL forward to a moderator. This works fine if the email is emailed directly to the DL email address itself but if the email is sent directly to the email address of an individual user it will be quarantined for approval which is not really what we want. Any ideas what's causing this?
1) you send from outside to the email address of the DL the email gets rejects (that's what should happen)
2) you created a transport rule to ask for moderation if an email is sent from outside the org to a user in a specific DL and this is what is happening no ?
" Require that all senders are authenticated" means that emails sent TO THE DL will not get accepted and not to its members . if you want no one to send an email to the members change your transport rule to
1) email sender is form outside the organization
2) recipient is member of specific DL
3) Reject the message with the enhanced status code
that should cover you