I sent this mail directly to Brett by mistake - sorry Brett. Here it is again:

On 02/06/11 20:16:25, Brett Parker wrote:

I'd almost bet if you google for the address it's already out there, but meh.

It's not.

...

All emails sent, using my standard mailer (Balsa), are directed through my ISP i.e relayhost= in the Postfix main.cf file and don't give any errors/problems. This current problem happens only when using /usr/sbin/sendmail directly which of course is what procmail does when forwarding a copy.

Which won't (neccessarily) send from a valid e-mail address, and the relayhost is (correctly) rejecting it.

So, you want to make sure that the sending address is correctly qualified, the server is checking that, and then going "hang on, that address doesn't resolve". -- Brett Parker

You are quite right of course. I discovered that when using sendmail directly with the -f option it does work.

I've now engineered a procmail recipe involving sendmail -f that also works. So all is now back to working the way it did before.

Thanks for your help and if I ever come across a chocolate teapot you'll be the first to know.