States may not completely ban (some certain state restrictions are permitted) lawyers from sending truthful and nondeceptive letters to potential clients. (see Shapero v. Kentucky Bar Ass'n (1988)
Remember that mail advertising poses less of a risk than face to face contact. You might be asking yourself, less of a risk of what? The risk that is contemplated is one of an invasion of privacy, or one of undue influence. The reasoning behind this is that lay people (potential clients) are not under any significant pressure when receiving a mailing advertisement, unlike face to face solicitation, where the lay person may feel very pressured.
In fact, Bolder v. Youngs Drug Products Corp discusses receiving mail/advertisement, and then easily being able to dump that mail in the trash. That receiving mail, and then having to dispose of that mail, is an acceptable burden that we as a society can accept in order to continue to provide constitutional protections to those who wish to advertise (free speech).
What you need to remember:
Rules of Professional Conduct 1-400 discusses the differences in a communication and a solicitation. This rule also tells us exactly what cannot go into any communication/solicitation, such as false or misleading information.
For further information see the following:
Shapero v. Kentucky Bar Ass'n (1988) 486 US 466, 473-474, 108 S.Ct. 1916, 1921-1922
Cal. State Bar Form.Opn. 1988-105
Bolger v. Youngs Drug Products Corp. (1983) 463 US 60, 72, 103 S.Ct. 2875, 2883
Zauderer v. Office of Disciplinary Counsel of Supreme Court of Ohio (1985) 471 US 626, 641-642, 105 S.Ct 2265, 2276-2277