okay i'll explain as best as i can...

i've got my mail server setup and running fine now mail comes in and out as expected. However, i've noticed a loop hole;
if a mail comes in with "rcpt to" and "mail from" the message is accepted and relayed on.

now i'm thinking this is wrong and i want to stop it.

i'm using exim and thought about putting a rule in that would do the following:

reject condition = if ($sender = $recipient)
condition = ! authenticated

can anyone confir if this sort of thing would work or if there is a better way to do do this ?

It looks like you are potentially an open relay. you should be using authentication on your SMTP server. I have never used exim, but here is a link that links to some documents on how to configure this for exim.