Will my patent protect me from overseas companies that emulate my product and try to sell to U.S. customers online through mail?

Your patent grants you the exclusive right to make, use, sell, and import any product that's covered by your patent's claims. A company overseas cannot, therefore, import a product that is claimed in your patent.

Only a patent attorney can tell you if an imported product infringes your patent. If it does, then your attorney will tell you of the mechanisms in place to prevent importation -- such as filing a complaint with the US International Trade Commission or in a federal court. If the importer's web host is located in the US then you can also send it a take-down notice. If the infringing product is being sold under your brand name then you can also seek trademark infringement remedies. If the infringer is using a domain name similar to a trademark you use you can also initiate a domain name dispute proceeding.

In short, you have the right to exclude the importation of infringing products. Stopping those products from actually coming into the country, however, is a real chore -- and an expensive one.