Okay,

I have struggled with sendmail for long enough now, it's time I got some help

When I try to send mail using a PHP script on the default RH EL 3 Setup, the mail just gets stuck in the queue (sendmail -bp), with "Deferred, host name lookup failed".

Why? Can anyone shed some light on this?

I found this very useful when I had the same problem: http://www.fraw.org.uk/cltc/document...tc-doc-04.html

I'm not 100% sure on the way that works.

It givesw loads of default IP addresses, but i'm not sure what I should put instead...

Also, will this allow me to send mail OUT of the system?

Have you changed your default sendmail configuration? If not you may want to check out the following line in the /etc/mail/sendmail.mc file:

dnl #
dnl # The following causes sendmail to only listen on the IPv4 loopback address
dnl # 127.0.0.1 and not on any other network devices. Remove the loopback
dnl # address restriction to accept email from the internet or intranet.
dnl #
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

and comment it out with:

dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

Then restart sendmail:

service sendmail restart

Please let me know if this works.

You have to rebuild the sendmail.cf first.

By memory, I believe ->
"m4 sendmail.mc > sendmail.cf"

Yes, I forgot that part. the .mc file says the following at the beginning:

dnl # This is the sendmail macro config file for m4. If you make changes to
dnl # /etc/mail/sendmail.mc, you will need to regenerate the
dnl # /etc/mail/sendmail.cf file by confirming that the sendmail-cf package is
dnl # installed and then performing a
dnl #
dnl # make -C /etc/mail

So after you update the sendmail.mc file, run the following:
make -C /etc/mail

Also, make sure you have the following packages installed:
sendmail
sendmail-cf

check with
# rpm -qa | grep sendmail