Because read receipts are a "hack" -- they're not supported nor were they ever intended in the internet email protocol, so Outlook and other programs that implement them are basically "hacking" a method to get them to work.
Kind of like buying a car, then asking, "How do I get it to make me a cup of coffee?" Sure, someone posted a tutorial on how to hack your car to make hot coffee off of the warmth of the engine, but it's not a "supported" configuration and is not part of the "car" standard. You car isn't even supposed to be able to make a cup of coffee... ergo, email was never intended to support read receipts (although they're semi-widely used, so people have the expectation of them).
Once you enter the Terminal command or commands that you wish, then the next step would be to set up a Mail.app rule. Head over to Mail.app's rules (I don't know where that is under 10.3 -- haven't used that OS in years) and create a new rule that is based upon the following (should be self-explanatory once you find where to create a new rule):
Code:
Description: Read Receipt Requested
if (all) of the following conditions are met:
(Disposition-Notification-To) (contains) (@)
Perform the following actions:
(Run AppleScript) (~/Library/Scripts/Mail Scripts/Return Receipt.scpt)
Then, download the code for the Applescript (right-click, download file, rename to "Return Receipt.scpt") and place it in the appropriate folder on your hard drive, defined above (home folder -> Library -> Scripts -> Mail Scripts).
Once that's done, test it and report back with any errors or where in the process it's failing and we'll try and troubleshoot it with you.