Faxes and E-mail Letters can take the form of faxed documents transmitted from one facsimile machine to another. Do offer to pay postage on materials sent at your request, or to pay for the return fax call. Many fax machines use thermal paper, an impermanent medium. Thermal paper isn't a concern in most business applications, because if not exposed to direct light, it will remain readable for a year or more. If, however, the information you receive on thermal paper has lasting value, photocopy it onto plain paper.
Faxes also come and go from computers, and are never actually printed on paper.
Since magnetic media are also impermanent, print out the faxes you want to keep and file them with your permanent records. E-mail is much more informally written than standard letters. Don't let this tempt you into failure to completely cite your sources in your messages. When another researcher e-mails you a request for information, consider the possibility that the message you're about to dash off may be forwarded around the globe to others interested in the topic. Millions of e-mail messages shoot through cyberspace every hour.