In Shapero v. Kentucky Bar Association, 486 U.S. 466 (1988), the Supreme Court held that a state can not ban a lawyer from sending targeted letters to potential clients in part because the invasion of privacy involved in receiving the unsolicited letter was minimal. As someone later put it, the trip from the mailbox to the trash can is a short one.
You would think the same could be said about a fax (technology that is not as common these days), but in a 2013 opinion,
the 7th Circuit found that unsolicited faxes sent by a Chicago lawyer to potential clients were advertisements covered by the Federal Telephone Consumer Protection Act, which imposes penalties for sending faxes without an opt-out provision. As a result, the lawyer was ordered to pay $500 for each of his 8,430 faxes, amounting to $4.2 million.
The case has been going up and down to and from the Court of Appeals since then and just recently it was reported that the Court has denied the most recent appeal. Go here for more details.