With compression in the application layer, an attachment will
be compressed by the original sender, and not uncompressed
again until by the final recipient.
In addition however the receiving UA will need to have compatible
capabilities with the sending UA. If this is not the case, the message may
get there, but will be rendered unusable. I suspect that this is the
problem with breaking software that Keith has been suggesting.
there are two kinds of brokenness that this threatens to introduce:
1. brokenness due to lack of backward compatibility
2. brokenness due to software bugs as a result of increased
complexity.
email is already too unreliable; the last thing I want to
do is make MTAs more complex than they already are (thus further
decreasing reliability) for a dubious benefit.
note that some approaches to compression are more reliable than
others - using a separate per-hop negotiated compression layer
on top of SMTP strikes me as more reliable than on-the-fly
translation of content-transfer-encodings. particularly if
the separate compression layer has built in integrity checking.
Keith