If you are writing about a specific family, include a group sheet with your sources carefully noted. Use a long, number ten, business size envelope with a one ounce, first class stamp affixed. Type, rubber stamp or print your name and address on the envelope in the area where the 'to' address goes and fold it in thirds to enclose with your letter. 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.