I use Informed Delivery for my PO box since I don't get much mail there. The worst thing about it is that the USPS uses the system to announce when there are new episodes of Mailin’ It! - The Official USPS Podcast.

They send the daily digest saying "You have 1 mailpiece arriving soon." Instead of the usual picture of the 'mailpiece', it's an image illustrating the episode. There is no physical mail corresponding to this alert, it's electronic junk mail. Spam. Ugh. There is no opt out for these apart from canceling the service entirely.