With the usage of POP3, the user always initiates the connection to the mail server.
Using a good mail filter for keywords, etc would prevent unnecessary words or phrases that could be prohibited from being transmitted.
As long as the mail server is on the internet and on the opposite side on the amprnet this should not be an issue to receive mail as the user is requesting the mail from the server and thus falls under 3rd party traffic. Same as dialing a phone patch to a 3rd party, the licensee initiates the connections.
My mail server has an option to have a list of approved senders as well as to require new senders to confirm their identity before mail will be released to the user.
Maybe some configuration or hybrid of the sort would work.
Best Regards
Elias Kd5jfe Louisiana Amprnet IP Coordinator