Mail Path MTU discovery?

Hi,

the mails I receive are getting bigger and bigger. 

I can remember to have configure a size limit
of 100k in sendmail some years ago, then it was changed
to 1M, and meanwhile some crazy people are sending me 
messages with video sequences of several minuts and
10-20 MByte. 

In a few years the latest movies will be ripped from 
DVD and sent as an e-mail.

I wonder whether it would be desirable to use similar 
algorithms as for IP routing for Mail Path MTU discovery.

Example: Before sending the 5 GByte E-Mail, a short control
message could be sent to the recipient, where every relay 
can reduce the maximum Mail size. Then the message is sent back. 
This would of course not be a guarantee, since the path can 
change every second. But it's at least a hint and could avoid
the 5 GByte to be sent just for generating an error message.

Hadmut