If by return receipt you mean the Return-Receipt-To: header then this
should not be implemented under any circumstances. Thankfully very few
MTAs handle this now because it is a serious security problem - for
example:-
* Say I had put that header on this message. However the address
I put in there was not my address, but your address. How many
return receipts would you be getting from this list? Say I had
copied the original message to some of the *big* lists.
* If I had put that header with a mailbox I own as the target I
would now have the subscription address of all the list members
(that would probably be a breach of EU Data Protection
legislation, although working out who is the guilty party would
be a problem).
* I write a spamming worm of some sort. Each message it sends has
a Return-Receipt-To: header aimed at an anti-spam organisation.
Return-Receipt-To: was a serious problem more than 10 years back. Its
not got any better.
Then do the following - (a) ignore Return-Receipt-To if it differs from "Reply-To" (b) ignore Return-Receipt-To if the messages is from a list (contains "List-Id") - most vacation notifiers already include such counter-measures and work without incident.
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