Business owners frequently deal with lawyers for a variety of matters. Having a law degree and being a lawyer are two different things; not every person with a law degree actually takes or passes a state bar exam to become a practicing lawyer. Address a lawyer properly by understanding the difference between having a law degree and being licensed to practice law.
TL;DR (Too Long; Didn't Read)
For a practicing attorney, you address them as "Esquire" or "Attorney at Law." For salutations, you can use "Mr.", "Ms." or "Mrs." followed by their last name.
The Juris Doctorate Degree
A Juris Doctorate, or J.D., is a law degree, meaning the person has attended and graduated from law school. This is similar to a psychology student attending graduate school to get a Ph.D. in upper-level studies. The J.D. alone doesn't make a person a practicing attorney, nor is getting the J.D. necessarily a requirement to taking and passing the state bar exam.
Addressing a letter to someone with a law degree but who isn't practicing law means recognizing the J.D. as you would any other advanced degree. For example, "Attn: John Smith, J.D." is the appropriate way to address the envelope, as well as the address block in the letter. The salutation in the letter would be, "Dear Mr. Smith."
Practicing attorneys have taken and passed their state's bar exam. While most practicing attorneys did attend law school and likely have a Juris Doctorate, the J.D. is not noted in correspondence. Instead, address a practicing attorney either as "Esquire" or "Attorney at Law." These are interchangeable, though most lawyers will use one or the other on business cards or correspondence, such as "Joe Mill, Esquire." If you don't know how the attorney refers to himself, choose either. If a business card, letter or website is available, choose the term used by the attorney himself. Note that "Esquire" can be abbreviated as "Esq." Salutations don't note the attorney status. "Dear Mr. Mill," is the appropriate salutation.
If addressing an invitation, letter or envelope to a couple, and the wife is a lawyer, her name is placed before his. For example, "Jane Smith, Esq. and John Smith." Standard protocol addresses the more credentialed individual first. If both have equivalent advanced degrees or both practice law, revert back to traditional formatting.
References to Business
Lawyers serve many different industries working in a variety of business structures. Some attorneys maintain solo law practices, while others work for corporations or government entities. When addressing an envelope or letter to a lawyer, the lawyer's name is followed by the law firm, corporation or governmental agency on the next line before the address.
Most organizations maintain websites that list the names and titles of key employees. Take the time to review any additional titles that may pertain to the lawyer you're addressing. For example, an environmental agency could list an attorney's title as, "John Smith, Esquire, Regulation Specialist."