I came THIS close to dropping my Chase envelope full of cash in a mailbox today (I was trying to juggle too many envelopes at once) and after I recovered from my stupidity, I wondered what one can do in such a situation??
Any helpful postal employee Dopers here?
Gee, I could have at least phrased the question in coherent English!! (not my day today)
Sorry about that…
It should have read:
HOW DO YOU GO ABOUT RETRIEVING SOMETHING YOU"VE DROPPED BY ACCIDENT IN A MAILBOX (and, I should have added, not necessarily mail)??
I’ve done that. Once I dropped my car keys in the slot along with the envelopes I was mailing. I just noted the time the box was due to be picked up, got back there 15 minutes prior to that, and waited. When the postal carrier showed up, I told him what had happened, gave a description of the item lost, and showed him my ID. He then opened up the box and handed me my keys back. This happens all the time. As long as the item lost dowsn’t have postage on it, they will usually give it back right there. If you accidently POST something you shouldn’t have, then, like on this board when you do the same thing…you are screwed. 
Your version uses ‘by’
I think it sounds better if rather than ‘by’ you add ‘ally’ onto the end of ‘accident’.
I was picturing electronic mailboxes, so your title was even more surreal to me.
Anyway - the only solution that comes to mind is to come back at collection time (which should be written on the mailbox) and rescue your item from the postman.
This happened to a friend of mine a few years ago. He mailed something and then realized he made a BIG mistake. I think he told his lawyer about it who then proceeded to scream at him over the phone. He went to the post office and spoke to the manager, who walked over to the mailbox with him, opened it, found the letter, gave it to him and never asked for ID. Go figure.
I realize the difference, but I did this once with my apartment’s outgoing mail slot. I’d bought something on eBay and paid for it with a money order. Not an hour after I dropped it in the mail, the seller e-mailed me with a corrected address. :rolleyes: So I wrote up a note describing my envelope and the situation, and asked the carrier if she could move the letter to my mailbox. She did.
You can sometimes retrieve stuff that you didn’t mean to mail or changed your mind about but you need to do it fast, like really, really fast. See the postmaster and describe exactly what you are looking for. I had to show ID, but at least I didn’t look like a complete fool except to the Postmaster and he didn’t care a bit.
“Hello. My name is Mr. Burns. I believe you have a letter for me.”
"OK. WHat's your first name Mr. Burns?"
"I don't...know."Several years ago, my parents were taking a multi-week trip overseas. So the morning they are to leave, on the way to the airport, they stop to mail payments on some bills. Into the collection box goes the bills, and the airline tickets. (Pre-etickets, so not having the papercopies would have been a real pain.) To make matters worse, it was after the last collection time on Saturday, so no more collections until Monday.
Thankfully, the local postmistress took pity on them, and came out to open the box for them.
Crowbar 
here in philly a kitten was put into a post box. a person who was going to drop in a few env. heard the kitten and got another passerby to go a few blocks to the fire station, while he stayed at the box. the firemen got someone from the nearest post office to come down and open the box. it was a sunday morning and it took a few hours, but the kitten was okay and the person who heard the kitten kept him.
i remember that he named him something connected to the post office but can’t remember exactly what.
i get a bit anxious dropping heavy mail in the post. i really listen to be sure there aren’t any cute critters in there before i drop it in.