[Exim] Not accepting mail from local nntp server

Hi,

I'm pretty new to exim so I'm probably just doing something daft, but I
can't seem to work this one out:

I'm setting up Exim 3.34, and it mostly seems to be working as expected.
However it's rejecting email posted by my leafnode nntp server (which is
running on the same machine).

My log file shows:

Mar 2 20:20:03 [sSMTP mail] smtp server didn't accept MAIL From,
replied "501 <news@_HOSTNAME_>: domain missing or malformed".
Mar 2 20:20:03 [leafnode] mailto: output of mta: /usr/sbin/sendmail:
smtp server didn't accept MAIL From, replied "501 <news@_HOSTNAME_>:
domain missing or malformed".

I have the following in my configure:

primary_hostname = mail.invisibles.org
qualify_domain = invisibles.org
qualify_recipient = invisibles.org
local_domains = "@:invisibles.org:[127.0.0.1]"
hosts_treat_as_local = "@:invisibles.org:[127.0.0.1]"
relay_domains = "@:invisibles.org:[127.0.0.1]"
host_accept_relay = "@:invisibles.org:[127.0.0.1]"
host_auth_accept_relay = *
receiver_unqualified_hosts = "@:invisibles.org:[127.0.0.1]"
sender_unqualified_hosts = "@:invisibles.org:[127.0.0.1]"
receiver_verify
sender_verify

Sending email from the command line from unqualified addresses using
'sendmail -t' seems to work okay...

Any idea why leafnode is unable to send email?

Thanks,
Tim