Chaplains provide pastoral services on behalf of religious and secular organisations. They may also hold other religious titles, military ranks or official positions that need to be considered when addressing them.
When addressing a military chaplain verbally, use "Chaplain" before his name regardless of rank or branch of service. Add the rank and branch in a written address, as in "Chaplain John Doe, Major, U.S. Air Force," and begin the letter with "Dear Chaplain Doe."
If the chaplain holds a religious title or doctorate in theology, use that title before his name, as in "Rabbi (or Father/Reverend//Pastor/Doctor) Doe" for a verbal address. In a written address, follow the title and name with "Chaplain" and the name of the organisation, as in "Rev. John Doe, Chaplain, Wesleyan University" and begin the letter with "Dear Rev. (or other religious title) Doe."
Address a chaplain for a branch of government verbally as "Chaplain Doe." In written communication, use his name followed by "Chaplain" and the government branch, as in "John Doe, Chaplain of the House of Representatives."