What is penalty for opening spouses mail?

Answer 1

Opening someone else's mail is a federal offense which can be punishable with a fine and possible incarceration or both, depending on the type, severity, and court discretion. For a better idea, you can contact the postmaster within your area.

Answer 2

There's no special penalty. Tampering with the mails is tampering with the mails.

In practice, it's exceedingly unlikely that anyone would ever be prosecuted for it. The general presumption is that in an amicable domestic relationship, it doesn't really matter whose name is on the water bill or who opens it, and it's perfectly legal for anyone to open someone else's mail if they've been given permission to do so. The spouse accused of tampering could simply say they had permission (either explicit or implied), at which point the court case turns into one big episode of He Said/She Said. Judges have seen that show before and aren't especially interested in a rerun.

In an acrimonious or divorce situation, it's a lot harder to claim permission, of course.