Whenever addressing another person, using manners and respect is never out of fashion. Regardless of your personal views on the military, the men and women who chose to serve their country did so to preserve your right to have your views. In most instances and circumstances of life, a formal address is appreciated as a gesture of respect. This is true for retired military personnel, as well as active personnel.
Addressing an Officer
According to the U.S. Code, Title 10, Subtitle A, Part II, Chapter 45, S 772, paragraph (c): "A retired officer of the Army, Navy, Air Force, or Marine Corps may bear the title and wear the uniform of his retired grade." This indicates that in speaking with a retired officer, it is proper etiquette to use the person's title. For instance, you might greet General Taylor by saying, "Good morning, General Taylor."
For most officer ranks, generally the lower ranks, the use of "Sir" or "Ma'am" is also acceptable. In the workplace, however, many retired personnel may prefer to go by Mr. or Ms. One exception to using the rank while addressing a retired officer is if the retired officer has taken a civilian job with the military. Then, while on the job, he would only be referred to by standard honorifics (eg. Mr. or Ms.) to avoid confusion with active military personnel.
A salute is a gesture of respect between fellow members of the armed forces. Therefore, civilians do not salute members of the military. And members of the military do not generally salute when either the senior or the subordinate is out of uniform. So if a uniformed, junior ranking member of the military encountered a higher ranking retired officer who is also in uniform, then a salute would be an acceptable form of address.
Addressing Enlisted Ranks
The same paragraph in the U.S. Code Note (c) does not refer to enlisted ranks. Therefore, the standard title of Mr., Ms., or Mrs. generally applies. However, you will often hear a retired NCO (non-commissioned officer), such as a sergeant, referred to by his rank, again out of respect.
The DOD (Department of Defense) recommends that mail addressed to any military personnel, whether retired or serving active duty, should be plainly addressed, using standard, civilian honorifics. However, many in the military prefer to have their ranks on the envelope.
In the case of retired officers, the standard is to write the rank, followed by the full name, followed by (ret). For example, an envelope might be addressed to General Samuel Taylor (ret).