I tried this in 10.4.5 with mail 2.0.7 and it doesn't work.

It lets me add the customer header but never displays it.

Sorry. that's rubbish. I set up a rule to change the background colour of messages requesting a read receipt and it worked, and the header was displayed.

The problem was that I am at a client's site and when I sent myself a message with a read receipt, the client's mail app has stripped it out of the message.

One problem though - reading this hint gave me the impression that I would be able to ask for a return receipt on outbound messages but I cannot. Have I misunderstood this hint?

I was obviously having a bit of a blond moment!

The terminal entry specifies the e-mail address that the read receipt will be sent to for all outbound messages!

D'oh!

This works although it is obviously a bit limited in that you can't do it per message.

Thanks for the hint though.