[exim] Strange problem with sending e-mail that has a hyphen…

I am running Exim on a Fedora server. Today I have been encountering
trouble sending e-mail to an address that contains a hyphen. E-mails
to this address from my gmail account work fine - but whn they
originate on this server they appear to be delivered to the SMTP
server - and then disappear. Here are a couple of log lines - 2
accounts at the same destination server -

     2008-05-17 23:02:28 1JxZAJ-0000Ad-M1 => not-real@???
R=lookuphost T=remote_smtp H=mxmta.sympatico.ca [67.69.240.19]
     2008-05-17 23:04:12 1JxZBz-0000Cj-KB => notreal@???
R=lookuphost T=remote_smtp H=mxmta.sympatico.ca [67.69.240.18]

Note - both of these accounts are uniqie, and do exist - but the
address with the hyphen disappears into thin air - no error message,
no bounce.

Any ideas what could be causing this?