I scoured OED-online for this word already, which my dad claims exists, but I think he's imagining things: a word that describes two letters, A and B, crossing each other midway, such that A was going to B and B to A. Let me know if this word exists, but if not, maybe sharing fancy words for postal events, like this one I got from the USPS glossary of terms, will make it all worthwhile:

à découvert - (French) Universal Postal Union term for open transit mail (mail that crosses U.S. borders while en route from one foreign country to another) that is in unsealed containers.

Also, "être à découvert" supposedly means to overdraft your bank account in French. The respondent below makes clear why à découvert applies to 'unsealed' containers.

Thanks all!

Literally à découvert means uncovered , unsheltered, not protected . Welcome.
As for the crossing, no idea.

Wow. I thought my query was specific! Can't help, but it's a neat concept; it would be equally neat to have a word for it.

What would you suggest? I would simply call them crossnotes.

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.

What would you suggest?

Letters that cross in the mail.

Pfh: I agree that if there isn't one there ought to be. If you have the patience, however:

http://onelook.com/?w=*&loc=revfp2&clue=letters+cross+mail

xpost ?

heh