Now to the point of this example: preventing unauthorized people from relaying SMTP mail through your system. First, keep in mind that you want to allow local users to relay SMTP mail. For instance, POP and IMAP users rely upon using Messaging Server to send their mail. Note that local users may either be physically local, in which case their messages come in from an internal IP address, or may be physically remote but able to authenticate themselves as local users.
You want to prevent random people out on the Internet from using your server as a relay. With the configuration described in the following sections, you can differentiate between these classes of users and block the correct class. Specifically, you want to block mail from coming in your tcp_local channel and going back out that same channel. To that end, an ORIG_SEND_ACCESS mapping table is used.
An ORIG_SEND_ACCESS mapping table may be used to block traffic based upon the source and destination channel. In this case, traffic from and back to the tcp_local channel is to be blocked. This is realized with the following ORIG_SEND_ACCESS mapping table:
ORIG_SEND_ACCESS tcp_local|*|tcp_local|* $NRelaying$ not$ permitted
In this example, the entry states that messages cannot come in the tcp_local channel and go right back out it. That is, this entry disallows external mail from coming in your SMTP server and being relayed right back out to the Internet.
An ORIG_SEND_ACCESS mapping table is used rather than a SEND_ACCESS mapping table so that the blocking will not apply to addresses that originally match the ims-ms channel (but which may expand via an alias or mailing list definition back to an external address). With a SEND_ACCESS mapping table one would have to go to extra lengths to allow outsiders to send to mailing lists that expand back out to external users, or to send to users who forward their messages back out to external addresses.