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By the looks of it:

There is a way to convince Mail in Tiger to always request a delivery notification (if the recipients' mail client supports that feature and the recipient has not turned them off)

Note: the following relies no undocumented features of Mail and I don't take any responsability if things break for you...

• Quit Mail
• Open Terminal (/Applications/Utilities/)
• Enter the folowoing text (make sure to keep all quotes exactly as is!)
defaults write com.apple.mail UserHeaders '{"Disposition-Notification-To"="your@address";}'
(of course, using your correct email address instead of "your@address")
• All future messages sent with Mail will now include the Disposition-Notification-To header which is the one asking for a return receipt.

If you wish to turn off this feature in the future, type the following in the Terminal:

defaults delete com.apple.mail UserHeaders

Andreas

I'm aware this says Tiger but maybe worth a shot?

Original location: http://discussions.apple.com/message.jspa?messageID=2297045
 
i have to add that i have tried this on Leopard with gmail and had issues-mainly that somehow the person that i was requesting the read reciept ws getting messages from someone else who was part of the message recipient-
 
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