Note that there's also the PIPELINING extension (RFC2920), which helps
a lot with *small* messages over paths with loss or large RTTs, by
reducing the number of "turns" in the SMTP conversation, and thus the
number of packets.
If you want to improve e-mail connectivity for people in poorly
connected places, it would be a good idea to add PIPELINING support to
your favorite MTA if it isn't there already (btw. I've done this for
Smail when this was my favorite MTA, and I think Ned has already done
the same for his).
For huge messages, PIPELINING doesn't help that much, but other people
have pointed to existing mechanisms that do.