While the email community has had some discussions about development
of a Requirements document (or two) there is no progress, as yet. So,
the direct answer to your direct question is: Not much anytime soon.
On the other hand:
1. MIME provides you, the sender, with a way to fragment the message
into several independent sections. You may want to explore sending
the IMR as a series of 32Kbyte message, since that seems to be the
email equivalent of IP's 576byte minimum-maximum.
2. The SMTP extensions working group is pursuing addition of an SMTP
command to allow a sender to determine the max acceptable size, before
sending. This would give the sender the option of fragmenting the
message only for sites that need it.
Along the lines of appreciating the half-fullness of a glass, rather than
its half-emptiness, I was astonished to see that 100K, rather than 32K,
was the limit at so many sites...