Address Extensions

[Daniel L. Miller]

I'm trying to implement a mail-to-fax gateway, and I thought address 
extensions might help me.  If I'm approaching this wrong please correct me.

I've already proven to myself that I can define a fax service in 
master.cf, add a fax transport mapping, and it works great.  However, 
this uses a host accessible only through my internal LAN - and I didn't 
want to expose my fax gateway to the wide world.  Selfish of me, I know.

However, I find that in order to accomplish one of my goals, which is to 
send e-mails to this fax gateway from Quickbooks, I have no choice but 
to expose the gateway in some fashion, as sending e-mails from a 
Quickbooks client involves the QB client sending the mail to Intuit's 
servers - and then out to the target.  Which means my fax has to leave 
the network and then come back.  Love the efficiency.

So...my initial thought was I'd have to create a public Internet name 
for the fax gateway, and apply some level of security to only accept 
submissions from Intuit.  However, I'm now wondering if I can accomplish 
the same thing by using address extensions instead of a different server 
name.  So I'd be sending emails to "1234567890+...@mydomain.com", and 
Postfix would then identify a fax is to be sent by the extension, 
translate that to "1234567...@fax.myinternaldomain.com", and process 
accordingly.  I would still have to protect the "fax" extension, but my 
thought is that the extension would be less likely to be probed than a 
published DNS name, and therefore be subject to fewer attacks.

Is there a HOWTO already written for something like this?  Or can I have 
a little guidance on what I need to study to implement this?
--
Daniel