Remove Return Receipt automatically if present from a mail and let the mail user know that the return receipt flag was set on the document by adding information text to the subject field (along with the original subject).
Create a private agent (when the agent is private, the refresh design which normally runs does not remove private design elements). Set the agent to run when "New mail has arrived". Select the Formula radio button and copy the following code into the formula area:
FIELD ReturnReceipt:= ReturnReceipt;
FIELD Subject:= Subject;
@If(ReturnReceipt = "1";
@SetField("Subject"; "****** INFO: DISABLED \"Return Receipt\" ****** " + Subject)); ""); "");