DuckCorp Dico

Found one definition

1.                 From The Jargon File (version 4.4.7, 29 Dec 2003):
                   
   
   
                   
                     
   posting
    n.
       Noun corresp. to v.: post (but note that post can be nouned).
       Distinguished from a ‘letter’ or ordinary email message by the fact that
       it is broadcast rather than point-to-point. It is not clear whether
       messages sent to a small mailing list are postings or email; perhaps the
       best dividing line is that if you don't know the names of all the potential
       recipients, it is a posting.