Is it legal to open other people's mail?

Answer

No. Federal privacy laws prohibits the opening of another person's mail. Title 18-Part 1-Chapter 83-1703 states in part that "Whoever, without the authority opens or destroys any mail or newspaper package not directed to him shall be fined under this title or imprisoned not more than one year or both." Obviously there can be large variances in how the law is applied.

Wrong! That law applies only to postal employees. Well, unless the question was posted by a postal employee, in which case....

See 463 F.Supp 128; 403 F.Supp. 1368;

Try 18 USC 1702. That one applies to civilians:

Whoever takes any letter, postal card, or package out of any post office or any authorized depository for mail matter, or from any letter or mail carrier, or which has been in any post office or authorized depository, or in the custody of any letter or mail carrier, before it has been delivered to the person to whom it was directed, with design to obstruct the correspondence, or to pry into the business or secrets of another, or opens, secretes, embezzles, or destroys the same, shall be fined under this title or imprisoned not more than five years, or both.