[exim] SMTP host or exim setting to send all emails

I have a site using joomla. Email can be automatically sent out to
people who register, or send us info via contact form, and we can send
newsletter to them.

I use SMTP server to process all the emails and set up the smtphost as
localhost. However, only emails to the domain names in same server can
be sent. Emails from other domains outside my server are simply bounced
back with the error: retry time not reached for any host after a long
failure period So the setting of localhost meant the emails sent could
be received within the domains in the same server.

I checked the FAQ 620 but the answer did not apply in this case.

What do I need to edit exim for the emails to be sent outside all the
domains in the same server or any suggestion?

Best regards
phyllis