I have now fixed the problem where mail relayed by exim was dropped somewhere because the (default) envelope-from address is not valid outside of my own system. (I broke the thread because I am sending this from my Debian box via exim :-))
Messages may (read: will) be dropped if they don't have a valid envelope-from address. By default the envelope-from is [EMAIL PROTECTED] where 'user' is the local user name and 'host' is this machine's name. Since this machine isn't in any DNS tables the default envelope-from address will be incorrect. To fix this, edit /etc/email-addresses to contain a line to rewrite the envelope-from to have the users "real" email address. I added dman: [EMAIL PROTECTED] to /etc/email-addresses and now the envelope-from is valid (for me anyways). I would need to add a similar line for all other users on my system, if they were to send emails. -D