Opening mail addressed to another person without prior permission is illegal. Permission, implied or express, is considered withdrawn if the couple is estranged or divorced, notes family lawyer Jeanne M Hannah.
Opening another person's mail, even a spouse's, is mail tampering, a prosecutable crime, explains Hannah. However, if the couple regularly opens each other's mail without objection, permission can be inferred. If permission can be reasonably inferred, it is difficult to impossible to prosecute someone for this kind of infraction. Intentionally tampering with another person's mail is a federal offense that is punishable with up to five years in prison, according to FindLaw.