—used as a conventional title of courtesy except when usage requires the substitution of a title of rank or an honorific or professional title before a man's surname
spoke to Mr. Doe
2
—used in direct address as a conventional title of respect before a man's title of office
May I ask one more question, Mr. President?
3
—used before the name of a place, profession, or activity or before an epithet (such as clever) to form a title applied to a male viewed as representative of the thing indicated