Interesting topic for discussion.
When I saw the movie the first time, I had a "hmmm wait a minute" reaction to Ennis receiving that card back stamped like that.
For one thing we'll have to put it down to the local custom, but I don't believe the PO ever would stamp any mail with the word Deceased. They either deliver it or return to sender marked as undeliverable. Although as said I'm willing to allow for localized customs being different.
Second, if the mail was returned by the PO, then that means that it was NOT delivered to the house. Whether anyone dies or not, the family still lived at that house and the PO would not do something like that with the family still living in the same place. They would naturally keep delivering the mail and leave it to the family to make notifications, etc. Again small town customs might allow for different behavior, but the PO is not supposed to act on "common knowledge" with the sanctity of the US Mails! They need specific directives, and forms filled out.
So to try to analyze this logically, it means that either the mail was going someplace other than the home, and so the PO did stamp it and return it, or the mail was going to the house and Lureen (or someone in the household) had to have stamped it, or refused it. Now IF the card got back to the sender (Ennis) and it did, then it HAD to have had a return address on it, thus either Lureen was lying, or someone else refused it, or the mail was not delivered to the home.
I've already said my piece about whether the mail was delivered to the home...PO would not stop mail or stamp any with family still there. So did Jack have a private PO Box? and if he did, and it was private, how did anyone find out that he was dead and close the box? Small town postmaster knowledge? Maybe, but they wouldn't be able to close the box until the lease ran out, without some legal papers presented by the survivors.....the lease would have had to have expired...i.e., the rent came due and no one paid it. But is there enough time for that to have happened? Could be, could have been at the end of a cycle (I have a PO box and it's paid either by the year, or the half-year...again I will allow for different customs in different locations)
So after turning this all inside out, for me the only satisfactory explanation is that Jack DID have a private PO box, and the rent DID come due and wasn't paid so the PO then returned to sender any mail that later come through, although as i said....I don't believe they would stamp it Deceased, but simply stamp it Undeliverable, but I'll overlook that bit of this scenario. ( I think the whole fact of a post card stamped Deceased was a fictional narrative device that was necessary.) Small town customs could create flexibility but the PO could NOT give the mail to anyone else, and also could not forward it without a directive to do that from the person (without getting into legal survivor status which would put us back into Lureen knowing). They have to deliver as addressed, or return to sender as undeliverable, or if that's not possible, sent to Dead Letter Office, where if it's an actual envelope some official will open it and try to determine if there is any way to get it back to sender.
If instead of the card coming back to Ennis, he just had no response, he would still have telephoned Jack I think to see IF something had happened to explain why Jack was not responding to him. So from that point, the Ennis Lureen interaction would still be the same (only Lureen would have had to preface her comments with the first fact that Jack was dead but otherwise it would/could have been the same.)
A lot of talk for a seemingly small point!
But very interesting to dissect and think over.
Jack in Maine