"couldn't reach the mailbox" message

Nothing. The postman (or woman in my case) just likes to think they're funny every once in a while. The mail always gets delivered as it should.

From the times that I've seen it in my game, it's just a text that doesn't have any in-game consequences.

Is there anything near your mailbox? I sometimes get that message after Harvestfest, when the gnomes have scattered seed packets everywhere (that's so annoying...), and I've also seen in in a couple of houses from the gallery where the mailbox is in a bush or something.

At first it bugged me but sometimes I get the message when I absolutely know the mailbox is easily accessible. I think we are being in game trolled.

Once, the mail carrier managed to "throw the mail in there" from the other side of the neighbourhood. I was impressed.

First time I saw that in my game, I roared with laughter. But, like you, it didn't seem to prevent the mail delivery, all the same. What's that slogan? Neither rain, nor snow, nor sleet nor hail can prevent the delivery of the U.S. Mail … or something close to that. LOL I had a Sim with the high Archaeology skill who was supposed to be receiving deliveries for her to inspect and validate. All I could think of was uh-oh, did the artifact get damaged? Not that I could tell. This message is just one of those little 'fun' moments in the game.

its supposed to be there when the mailbox is wrongly located and not routable (things in front of mailbox etc) so sims can't get to it but lately its showing up at random times in apartments when there is nothing delivered
which is a bug

"GalacticGal;c-17197850" wrote:
What's that slogan? Neither rain, nor snow, nor sleet nor hail can prevent the delivery of the U.S. Mail

And in my dad's case being drunk didn't stop him either so there really is no excuse the Sims can't deliver the mail :p

I get this message regularly and I can’t figure out why. I swear it’s not a pathing issue. I’ve tried placing the mailbox out in the open (no moveobject cheats) and it still happens.

This means your mailbox isn't reachable for the mail carrier to do the job , like you delete mail box without noticing while building, something blocks the way to the mailbox, this including your sim is interacting with the mailbox while the mail carrier is about to deliver. Or your mailbox is inside your fence and you lock the fence's door, which makes it impossible for the carrier to enter and deliver. As long as that message keeps showing up, it means something must be wrong about the progress of delivering. Once you see the mail carrier show up and do put something into the mailbox, that means it's fixed. And you can tell who the mail carriers are easily because of their unique outfit.
But still, sometimes I do get the message even though my mailbox is fine. If this happen, maybe it's the NPC who plays the mail carrier has issues like glitch. Try to delete that NPC to generate a new one. If still not working, it could narrow down to a bigger in-game problem. Try to repair the game to see if it helps. If still not, go to report a bug for mail carrier not doing their jobs.