H.E. is a short written form of His Excellency or Her Excellency. You put it before the name of an ambassador — and sometimes a governor or another very high official — on envelopes, name cards, and formal lists. It is a title of respect, stiff and diplomatic; newspapers and embassy pages use it, friends never do. Though actually you almost never say the letters “H.E.” to the person’s face. In the room you say “Your Excellency”; H.E. is for paper. Which word it expands to, His or Her, follows the person, not the job.

閣下（大使等外交官頭銜的書面縮寫；當面則稱 Your Excellency）