I worked for Royal Mail in the U.K.
We put 1 in maybe every 1000 or so in a box to go to volunteers and the rest are destroyed and recycled. (SchezwanSaucey)
I can answer this from a UK perspective. Royal Mail has an official Santa address, but any letters addressed to Santa, even if it’s just a badly scribbled SANTA in child’s handrwritting gets handsorted and receives a Christmas card from Santa back. They even do Welsh versions! And there’s a different card if the reply comes after Christmas due to postal dates. They’ve been doing since 1964 and last year they received around 600k letters. Only catch is, it needs a return address. (meeeow)
I ‘misread’ the address and forward it to Satan (BaieMarba)