How do I let internet email through to a hidden group?

I have used the “Who can read this document” under the security tab to hide a user group from the rest of the users in the domain. However, I need to be able to let all incoming email from the internet to be allowed to send email to that group. How do I get it to allow that?

Subject: the mail server needs access

Give the mail server, or LocalDomainServers, access and you should be set.