Think you've got too many IM and twitter accounts, email and snail mailboxes?
Spare a thought for dogs. They have a pee-mail inbox at every tree, shrub, lamp post, bin, etc. etc. in a 5 block radius (plus a few stowed away at holiday destinations) and they just have to check each and every one, each and every time they go past. Not negotiable.
Okay, so pee-mail doesn't really exist, but it makes us humans feel better to anthropomorphise their behaviour (just as we do in so many picture books) just so that we can understand it ourselves.