Trust me making a change at the Post Office doesn't always work like it should.
On our last move, some mail I received, other mail got left at my old house, some mail was sent back to the sender - it was awful trying to get my mail. And this is after I went to the Post Office and put thru a forwarding address.
You still don't open mail that doesn't belong to you -
Since everyone on the DIS has opinions about things, I figure I'd post this situation here and ask my fellow DISsers....WWYD?
We bought our new-to-us home in June (well closed in June). For two weeks we were at the house every day cleaning what the previous residents left behind (they pretty much packed and left and did not clean).
So as we went back and forth from our old house to the new one each day for work, we'd bring a few boxs and misc odds and ends to try and get as much stuff as we could out of our old house before our "big move" day of June 26.
Anyway, during those two weeks, we would get the mail at the new house. Most of the mail was for the previous residents, we set it in a pile to deal with later.....planning to write "return to sender" or "no longer at this address" on the envelopes and put it back in the mail box.
Well, we never did. Today I found the stack (had been moved form place to place as we unpacked) and went through it. Everything that looked "important" I wrote on the outside envelopes "RTS" or "NLATA" but the rest I trashed. Then I saw the card. In the pile was a card -- a birthday card. I thought to myself "Oh man, I should have sent this back sooner so it could have been returned or forwarded to the recipient" I started to trash it, but a voice in my head told me to open it (after all it was very late) in case there was a check in it and I could mail the check back to the sender with a note that the family had moved.
Apparently, the card was for a small child and inside there was a $5 bill.
So I ask......WWYD with the $5.
Yes it is. It was legally delivered to their home. They own the property. They can do whatever they want with it.
TITLE 18--CRIMES AND CRIMINAL PROCEDURE
PART I--CRIMES
CHAPTER 83--POSTAL SERVICE
Sec. 1703. Delay or destruction of mail or newspapers
(a) Whoever, being a Postal Service officer or employee, unlawfully
secretes, destroys, detains, delays, or opens any letter, postal card,
package, bag, or mail entrusted to him or which shall come into his
possession, and which was intended to be conveyed by mail, or carried or
delivered by any carrier or other employee of the Postal Service, or
forwarded through or delivered from any post office or station thereof
established by authority of the Postmaster General or the Postal
Service, shall be fined under this title or imprisoned not more than
five years, or both.
(b) Whoever, being a Postal Service officer or employee, improperly
detains, delays, or destroys any newspaper, or permits any other person
to detain, delay, or destroy the same, or opens, or permits any other
person to open, any mail or package of newspapers not directed to the
office where he is employed; or
Whoever, without authority, opens, or destroys any mail or package
of newspapers not directed to him, shall be fined under this title or
imprisoned not more than one year, or both.
(June 25, 1948, ch. 645, 62 Stat. 778; May 24, 1949, ch. 139, Sec. 37,
63 Stat. 95; Pub. L. 91-375, Sec. 6(j)(16), Aug. 12, 1970, 84 Stat. 778;
Pub. L. 103-322, title XXXIII, Sec. 330016(1)(B), (G), Sept. 13, 1994,
108 Stat. 2146, 2147.)
Yes it is. It was legally delivered to their home. They own the property. They can do whatever they want with it.

Let's say someone sends something to you, with your name on it, but inadvertently writes your address at "123" instead of "132". Does that mean that the residents at "132" get to keep your mail?Yes it is. It was legally delivered to their home. They own the property. They can do whatever they want with it.

It was legally delivered to their home. They own the property. They can do whatever they want with it.
United States Codes (Federal Law Register)
Sec. 1702. - Obstruction of correspondence
Whoever takes any letter, postal card, or package out of any post office or any authorized depository for mail matter, or from any letter or mail carrier, or which has been in any post office or authorized depository, or in the custody of any letter or mail carrier, before it has been delivered to the person to whom it was directed, with design to obstruct the correspondence, or to pry into the business or secrets of another, or opens, secretes, embezzles, or destroys the same, shall be fined under this title or imprisoned not more than five years, or both
Sec. 1705. - Destruction of letter boxes or mail
Whoever willfully or maliciously injures, tears down or destroys any letter box or other receptacle intended or used for the receipt or delivery of mail on any mail route, or breaks open the same or willfully or maliciously injures, defaces or destroys any mail deposited therein, shall be fined under this title or imprisoned not more than three years
TITLE 18--CRIMES AND CRIMINAL PROCEDURE
PART I--CRIMES
CHAPTER 83--POSTAL SERVICE
Sec. 1703. Delay or destruction of mail or newspapers
(a) Whoever, being a Postal Service officer or employee, unlawfully
secretes, destroys, detains, delays, or opens any letter, postal card,
package, bag, or mail entrusted to him or which shall come into his
possession, and which was intended to be conveyed by mail, or carried or
delivered by any carrier or other employee of the Postal Service, or
forwarded through or delivered from any post office or station thereof
established by authority of the Postmaster General or the Postal
Service, shall be fined under this title or imprisoned not more than
five years, or both.
(b) Whoever, being a Postal Service officer or employee, improperly
detains, delays, or destroys any newspaper, or permits any other person
to detain, delay, or destroy the same, or opens, or permits any other
person to open, any mail or package of newspapers not directed to the
office where he is employed; or
Whoever, without authority, opens, or destroys any mail or package
of newspapers not directed to him, shall be fined under this title or
imprisoned not more than one year, or both.
(June 25, 1948, ch. 645, 62 Stat. 778; May 24, 1949, ch. 139, Sec. 37,
63 Stat. 95; Pub. L. 91-375, Sec. 6(j)(16), Aug. 12, 1970, 84 Stat. 778;
Pub. L. 103-322, title XXXIII, Sec. 330016(1)(B), (G), Sept. 13, 1994,
108 Stat. 2146, 2147.)
Get a grip people!!!!
The previous owners had a responsibility to make the change at the post office. That was their error.
Don't worry, OP, you didn't do anything wrong. You broke NO laws. Contact the post office for the mailing address of the previous owner. A quick note explaining what happened with the card money and other mail should more than suffice.

I would have put ALL the mail right back in the mail box, rubber band it together if necessary and put a sticky note on the mail, please forward. I certainly wouldn't open any of it, let alone a card that probably contained money. Poor kid's mom will probably be posting on a message board somewhere how Grandma forgot his birthday and never forgets anyone elses too.