[exim] Dealing with Yahoo mail RFC violations

Hello,
Yahoo.com set a maximum message size of about 6-7M. When the data hits the maximum size yahoo just drop the connection.
So although the message is permanently rejected exim will see it as a temporary problem.

I have set a custom retry for yahoo so that the messages do not retry so much and time out sooner. But I am still getting large (by definition) messages stuck in a continual retry cycle which is affecting performance.

Ideally I would like to set a size limit for the yahoo.com domain so that I immediately reject any mail > 5M with a yahoo recipient (and possibly others throwing the baby out with the bath water etc)
I could set a var in check_rcp for yahoo and check the size in check_content.

Is there a more sensible way to do this, or a better way all together to deal with the problem.

TIA
Stuart.