[Exim] Problem with dots in usernames....

Hi,
I'm trying to send an email via exim to a (hypothetical)
dominic.400@???...

Exim doesn't seem to want to know about this address and just keeps it
held in line without sending - my exim configuration is ok for others
but always fails if there's a 'dot' in the username portion of an email
address.

What do I need to do to my exim.conf to allow for this case??

  Regards
     Dan