Earth is out of bounds, according to the Shadow Proclamation so the intergalactic mail service probably wouldn’t stop there even if it existed. Which it might, for all Donna knows; she’s never got around to asking the Doctor, there are always too many other questions to ask and no time to ask them. It’s hard to say much of anything while running.
There are postcards though, sort of, so she gets what she can when circumstances allow; pictures of the places she’s been. If she can’t mail them, she’ll take them home to Granddad herself. She knows he’ll love them.