I wonder if they manually check all the letters...would take too much work I'm thinking, so how can they check for spam or bots that auto send lots of them with

I wonder if they manually check all the letters...would take too much work I'm thinking, so how can they check for spam or bots that auto send lots of them with spammy messages? (you could send varied spam messages as well) EDIT: didn't read the part about 1 card only