The usual message includes Shergold's address and a plea to send postcards. The Shergold story was so common on the Internet several years ago that Gene Spafford, a professor at Purdue University, wrote a FAQ file about it. Craig Shergold exists, was diagnosed with terminal cancer, and actually earned the postcard record back in 1989. Shergold is now in his teens, but the postcards keep flowing in, much to the consternation of the British Postal Service and the hospital at which he was treated. Guinness, abashed by the chaos it helped to bring about, will no longer accept record attempts in that category.
Most urban myths are, by their very nature, at least superficially believable.
The organization that supposedly originated the message is not one you would expect to be involved in such things. If you are still unsure after trying these approaches, consider posting to alt.folklore.urban. But check the newsgroup's FAQ first. Many urban myths cause no material harm in and of themselves. Some myths unjustly damage the reputation of a specific business or person.