Hi,
I guess we should accept light speed as a limit. That's not the question. The question is rather: which protocols are appropriate for IM in the face of light lag. And the answer is: protocols without round trips. In this respect Jabber is better than SMTP, because it does not require a round trip on the app level protocol to deliver a message. Both break at the TCP level unless TCP is modified, which is a hobby of Vint (I am the father of the internet) Cerf. Simply speaking, if there is a SI layer 4 solution, then IM wins against Email. But on the other hand I am sure that there will be a no-round-trip version of SMTP, which essentially is an IM protocol then.
_happy_coding()
hw