Well, if they're using systems like Amazon SES for delivery, they might've been trying to save some processing time and bandwidth by combining multiple recipien

Well, if they're using systems like Amazon SES for delivery, they might've been trying to save some processing time and bandwidth by combining multiple recipients in one email (this is a perfectly normal way to send group emails). The key, though, is to put the extra recipients in the BCC field. My speculation is that this was a rookie mistake (maybe an intern or new hire tasked with the Christmas email?), or maybe Pingdom isn't used to sending marketing/mass emails. (Most of their emails are transactional, at least as far as I know).