18 USC Section 1702 prohibits the obstruction of the delivery of mail. However, your opening your husband's mail is unlikely to result in a federal prosecution for practical reasons, unless you use the contents of such mail for an illegal purpose.
Joshua Sabert Lowther, Esq.
NATIONAL FEDERAL DEFENSE GROUP
[email protected]
http://www.NationalFederalDefense.com
866.380.1782