"turn authentication on" -- a simple how to please!!

[David Southwell]

My mailserver uses postfix and has a number of virtual domains. I am getting 
the following difficulties on delivery of legitimate emails to remote 
addresses failing with a request to tun authentication on. I am comparatively 
new to managing mailservers. Could someone please tell me what I need to do. 
My searches on google seem to give me long explanations of what is meant to 
happen but I cannot find simple instructions on how to fulfill the 
requirements!!

dns1# fgrep abc /var/log/maillog
Sep  7 17:01:59 dns1 postfix/smtp[86489]: 179BE34D41D: 
to=<competitions at bristolphoto.org.uk>, 
relay=mail.abc.org.uk[xxx.xxx.xxx.xxx]:25, delay=2.2, delays=2/0/0.15/0.03, 
dsn=5.0.0, status=bounced (host mail.abc.org.uk[xxx.xxx.xxx.xxx] said: 550 
Please turn authentication on (in reply to RCPT TO command))
dns1#

Thanks in advance
david