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