Is federal trademark registration valid outside the United States?
In a sense it is. A federal trademark covers all 50 states territories and possessions of the United States, so a federally registered US mark is effective in Guam for example. But to that extent then no, if you want to protect your mark in Canada you need a Canadian trademark. There are other community marks in Europe for example but generally speaking whatever country you want a trademark in you have to get a trademark from that country.