Originally Posted By: Zed
I wonder if the post office has a specific professional-jargon term.


I wouldn't think that the post office would even notice that it had happened. Possibly in a small town post office someone might note that person X had received mail from person Y very shortly after person X had sent mail to person Y but even then I don't think it would be considered extraordinary enough to have a term for it. It happens in email all the time and I personally have never noted it with anything much more than a comment on ships passing in the night.