Dave;
Mail relaying is different from Mail gatewaying. A gateway tries to do a
constructive job of destroying information. If it didn't have to destroy
information, the services on either side would be semantically identical.
Hence, it has to get into the content and muck with it. A relay doesn't.
A relay is identical to a router.
I agree. What you call relay is a network layer relay and is identical
to a router. But, don't forget that, just as a router decrement TTL in
a packet header, your relay adds "Received: field" in a message
header. Both does not look at nor touch the content, of course.
It is surprising that even we can sometimes agree on layering related
issues. :-)
In particular, a gateway looks like a user agent (i.e., a 'data in your
platform-specific formats') so that the message goes through the logical
sequence:
Your gateway is an application layer relay.
Masataka Ohta