[imp] relaying through server with horde5 and remote mail clients

[peter lawrie]

Hi
I have set up a RedHat 6 server with Horde 5, it works correctly now (after
much headscratching) for users using imp for their mail.
However, if using an external mail client (I have been testing with
thunderbird 17.0.4) -
I can send and receive messages from horde / imp to users on external
domains and also
I can send mail from Thunderbird to imp users on the server but not beyond
to the external domains as the server will not act as a relay

Horde is using PAM on Dovecot to authenticate. ( dovecot-2.0.9-5.el6.x86_64
)
Sasl appears to be running

00:00:00 /usr/sbin/saslauthd -m /var/run/saslauthd -a pam -r

Most of the howtos I have tried to follow assume a single local domain, but
with virtualmin and horde one can have multiple domains
I have been through the dovecot howtos several times, trying to modify
main.cf to permit relaying
I don't want any bells and whistles, just a configured main.cf which will
allow this.
The server currently has 24 domains configured, most of which will
eventually require mail relay.

Peter Lawrie