The US Patent and Trademark Office (USPTO) has posted a warning on its website regarding solicitations that appear to be official notices (Click here for a sample). The notices come from private companies that are not associated with the USPTO.
The notice is typically made to resemble an official notice from the USPTO by including the trademark serial or registration number, classes, filing dates and other information that is publicly available on the USPTO's database. Most of the notices appear to be an invoice stating an amount "due". If you read the fine print (if it is even included), the fee is not for a legitimate government fee, but for legal services, trademark monitoring services, Customs recordation services or inclusion in a private registry.
Trademark owners who receive such a solicitation may file a complaint with the Federal Trade Commission at www.FTC.gov. The Trademark Office also provides an e-mail for reporting misleading communications at TMFeedback@uspto.gov.
The Trademark Office notes that all of its correspondence is from the “United States Patent and Trademark Office” in Alexandria, VA (if by e-mail, from the domain @uspto.gov). If you are represented by an attorney, most likely all government fee notices will come from your attorney.