Used stamps exchanges.

I received one that doesn't have a return address, but said ES-476. Trying to register that ID told me that it was for someone else.

4376 is for someone else.

try 4376

Try contact member "francescpf". He often uses barcodes. Maybe he is the right one.

The address was "ES-476 x 100" and the next lines were my name and address. They were cut out of a computer printout, so there is no possibility that I just misread the digit.

Trying all ten digits before or after doesn't work.

The envelope had a machine barcode (not a stamp) reading "Correos/Carta Ordinaria/Internacional/S'Arenal" (which is Spanish) and was postmarked 07/07/26.

Letter from Spain. ID must be four-digit. Try a bit :)