[Exim] HELO syntax check at RCPT

Greetings,

Is it possible to enforce the HELO/EHLO syntax check at RCPT time instead of
HELO time? My logs are starting to fill up with junk from syntactically
invalid hosts (yes those stupid underscores), that keep retrying many, many
times.

I do not wish to accept from these hosts. I just want to give them a 5xx at
RCPT time so they will (hopefully) stop retrying.

I probably just need more coffee. Any suggestions?

Regards,
Jim Roberts
Punster Productions, Inc.