Gateways should translate embedded messages, but should retain the original
form, by placing the old and new forms within the Multipart/alternate
structure.
Clearly the logical extension of the "if you don't like this one, try
that one" of multipart/alternate.
The only negative to this that I can see is that messages
will always get large.
It seems to me that, since the average bandwidth of the Internet is
lots higher than the average bandwidth of any of the WANs attached to
it, this is a *big* negative. Let's see, one takes a megabyte-sized
picture (a reasonable number, and makes calculations reasonable),
converts it to base64, ending up with circa 1.33 Mb, and then adds the
original megabyte back in. I don't really want to contemplate that at
9600 baud if I can help it, much less something slower.
The other problem is that, if the remote network operates with
radically different character set conventions--e.g., it requires 7bit
characters and uses the 8th bit (maybe in the middle) for parity--
"retain the original form" may not be a plausible option.
--john