Those who plan on sending Christmas gifts to the United States this holiday season may want to think twice now that parcel delivery companies are restricting U.S.-bound mail at the behest of airlines complying with new U.S. counterterrorism measures.

The extent of the restrictions varies with each company, but U.S. aviation authorities are expected to maintain them for the time being, throwing a kink into the gift-giving season as 2010 prepares to wrap itself up.

Below are a few questions and answers on the new mail restrictions.