[Exim] HELO syntax checking in Exim 4

Hi,

Ran Exim4 for the first time today in a production environment on one
of our main inbound MX servers and found that syntax checking of
HELO/EHLO appears to have changed somewhat between Exim3.35 and Exim4.01.

Exim 3 appears to have allowed underscores in the domain name in a HELO.
Exim 4 appears to not allow underscores in the domain name in a HELO.

A quick check of the RFC seems to indicate that Exim 4 is the correct
behavior.

However, the reality is that a lot of hosts seem to be using
underscores in helo domains.

Exim 4 appears to only allow me to turn off helo syntax checking
completely with the "helo_accept_junk_hosts" item. However, I really
don't want to do that either.

Is there a way to change the regexp pattern for helo domain checking
without having to completely disable it?

Thanks,

Dean Brooks
dean@???