🚀 go-pugleaf

I think the differences are much more distinct.

A message switch will not translate the message or address.  It will merely
pass it on as fast as possible.  Commercial message switches are able to
handle incredible throughputs and are typically based on proprietary
message formats.

A gateway will typically translate:

    1. the message encoding
    2. the address(es) of recipients(s) and originator
    3. facilities and specific features

In addition, a gateway has to "talk" both communications protocols used
on the two sides of the gateway.

A gateway is very complex (it has to know both message formats intimately)
whereas a message switch can be quite simple and typically only would
have to know about one message format.

An X.400 MTA is a message switch - except it can also add value in some
cases.

Gateways are the scourge of the industry, but they also allow implementations
based on proprietary and de facto standards to exchange messages with other
systems.  In the ideal world every implementation would be able to speak
a common protocol which would eliminate gateways.  Alas, the world is not
ideal and users will have to spend a lot of time managing gateways for many
years in the future.

Cheers,                             ....Erik.