Law enforcement sources tell us Vanessa Trump was exposed to the substance Monday morning. The letter -- which we're told said something along the lines of Jr. being an awful person and getting what he deserves -- was sent to her mother's apartment in Midtown Manhattan.

We're told Vanessa and 2 other people were taken to a hospital as a precaution. The substance itself was tested and NYPD says it's non hazardous. The envelope was reportedly addressed to Trump Jr., although it went to his mother-in-law's home.