The Postal Bulletin (first published as The Daily Bulletin of Orders Affecting the Postal Service) is a vital source of information for the researcher looking for information on U.S. Post Office Department (and later USPS) operations and policies. Its use has been limited, though, due to insufficient indexing and the necessity of finding a library holding the bulletins in print or microfilm.
Now, all of the Postal Bulletins from 1880 to 1971 are available and searchable online at www.uspostalbulletins.com.
This monumental project is led by Tony Wawrukiewicz and supported by the Military Postal History Society, Mobile Post Office Society, Postal History Society, Universal Ship Cancellation Society, United States Stamp Society, and individuals.
You can read more about the project, included step by step search instructions, in an article by Wawrukiewicz in the May 2012 U.S. Specialist.