Step 1 : Introduction to the question "Which of these U.S. states has a postal abbreviation whose letters are not in alphabetical order?"
..., three months after the Post Office introduced ZIP codes. Michigan’s postal abbreviation (MI) is not in alphabetical order. The remaining states of Mississippi (MS), Minnesota (MN), and Missouri (MO) all have two-letter abbreviations that are in alphabetical order
Step 2 : Answer to the question "Which of these U.S. states has a postal abbreviation whose letters are not in alphabetical order?"
Michigan:
The United States Postal Service has assigned a two-letter abbreviation to each of the 50 states. Modern two-letter abbreviated codes for the states and territories originated in October 1963, with the issuance of Publication 59: Abbreviations for Use with ZIP Code, three months after the Post Office introduced ZIP codes. Michigan’s postal abbreviation (MI) is not in alphabetical order. The remaining states of Mississippi (MS), Minnesota (MN), and Missouri (MO) all have two-letter abbreviations that are in alphabetical order.