Trademark can "exist" several ways. The first, and most common, is simply by usage of the mark. This is common law trademark, and is enforceable in court. "Use of the mark" is the touchstone. People will use the indication "TM" to indicate an intention to use the mark in trade.
The second way trademark can be asserted is via state registration. "Use of the mark" is the touchstone here as well, with the state registration being notice to those in the state.
The third for is interstate trademark, which is what the USPTO registers (also as notice to would be infringers). Federally registered trademarks require use in interstate commerce, and are noticed with "R" in a circle.
Just saying, if you make or use the mark, the registrant still has a viable legal claim against you. All he's lost is a presumption that attached by way of having the mark registered.
Thanks for the info Counselor.