The post office is launching a new barcode on first class mail that will enable the sender to track mail through the system:

With the new bar code, companies will be able to track mail delivery and know when their customers got a bill, solicitation or product, and the Postal Service will have another way of checking that mail is being delivered on time.

Companies also will be given a chance to buy data collected by the post office that will give them insights into how customers respond to advertising and marketing. A company, for instance, can buy a television or newspaper ad to tout a new product, follow up with an announcement in the mail and get a sense of how well the ad is connecting with customers.

So now the government will have a database of who sends mail to whom. Of course, there's no discussion of this in the news article.