Wrong post?

A so-called defector (post that is obviously not intended for you, e.g. only similar road as your other house number or the like) you must not actually open, but should throw it into the next mailbox.

Now that you’ve already opened it and noticed the error, of course, it would seem like you could bend the whole thing again:

• If the letter is simple for a neighbor who is also called Müller, but living in another house of the same street, I would just cuff with the neighbor, leave the letter right there and briefly explain that you unfortunately get his post, but only after opening notice that it is not for you.
• If the effort is higher (street in another district or possibly completely different town), I would add the letter again, pass the address and write “retour to sender -> delivery to wrong address” on it and throw it into the next yellow mailbox. Why not just “Irrläufer” but back? Well, often there are deadlines to observe, and so the sender has at least the chance to know that something went wrong in delivery and he has to send his letter again with a new deadline. The “original recipient” can’t do anything about it if it’s the postal company, and he gets the mail later.
• If you want to make the effort, you can also contact the sender and explain the situation to them (namely that you have mistakenly received foreign mail and that only after opening). However, debt collection offices are often quite “consultant” because of course someone could use it as an excuse.