BULK MAIL vs JUNK MAIL

BULK MAIL
Bulk mail broadly refers to mail that is mailed and processed in bulk at reduced rates. The term does not denote any particular purpose for the mail; in particular, it is not synonymous with "junk mail."
As an example, in the United States, the United States Postal Service defines bulk mail broadly as "quantities of mail prepared for mailing at reduced postage rates." The preparation includes presorting and placing into containers by ZIP code. The containers, along with a manifest, are taken to an area in a post office called a bulk mail entry unit. The presorting and the use of containers allow highly automated processing of the mail, both in bulk and piecewise, in processing facilities called bulk mail centers (BMCs).

JUNK MAIL
Although bulk mail, junk mail, and admail are, strictly speaking, not synonyms, the terms are used in common parlance to refer to advertising circulars, free trial CDs, pre-approved credit card applications, and other unsolicited merchandising invitations delivered by mail to homes and businesses. The term "junk" comes from the fact that in the majority of cases mail is discarded before it is opened by the recipient. Bulk mailings are a particularly popular method of promotion for businesses operating in the financial services, home computer and travel and tourism industries.

UNDER CONSTRUCTION
source:wikipedia