A lot of stuff happens because of the Portal. Not just odd arrivals. Some cool, some not so cool, some downright dangerous.
If you get a letter from Father Christmas, it's pretty much legit. Maybe not the literal Father Christmas bit, but you'll get just about anything you ask for as a prezzie.
[Doctor (12)/Rose] I'd appreciate that. The name's fallen out of the collective memory now, and I hope it stays that way.