At a conference recently, the Commissioner for Trademarks told the audience that the best way to help the USPTO combat the proliferation of scam mail targeting trademark applicants and registrants is to report them to the Fair Trade Commission (FTC). Such reports are apparently useful to help the government gather volume of the scams. So, before trashing the scam mail, please file a complaint with the FTC to help combat these fraudsters.
Here’s how I recently filed one such complaint regarding World Patent and Trademark Register (WPTR):
Begin by visiting https://www.ftccomplaintassistant.gov/. 
Choose: “Rip-Offs And Imposter Scams”
Choose “Continue” at the bottom right
Choose “Continue” again
Input the date the scam was received, and the fact that it was via “Mail”
Input the amount of money sought by the scammers
Provide the additional details requested about who the scam was sent by
Provide your information
Summarize the nature of the scam, and the effort to look like official government communication, the worthless nature of the publication or directory, and other details.
Submit the complaint.
Unfortunately, the FTC site does not request a copy of the material received or allow for any way to upload a scan of the letter.
Also report scam mail offers to the State’s Attorney General. If their return address is listed in Florida, then report to the Florida AG here: http://www.myfloridalegal.com/pages.nsf/Main/E3EB45228E9229DD85257B05006E32EC
If the return address is New Jersey then report it to the NJ Attorney General, etc.