How is someone who is retired from the USMC addressed?

Retired Marine Corps personnel can use their ranks rather than civilian titles if they prefer. To address a retired USMC member correctly requires knowing what his personal preference is, as well as knowledge of his former rank.

For formal correspondence, using the rank and service of the person in question is the correct manner unless he has adopted a civilian title for professional or academic purposes. This is accompanied by a notation of retired status in the form "Major John Doe, USMC, Retired." In speech, asking what the person wants to be called is easiest and avoids confusion or offence.