RFC 2076, Common Internet Message Headers (http://tools.ietf.org/search/rfc2076),
has some discussion regarding the Precedence: bulk setting. This RFC was dated
February 1997. I did a google search to see if this standard was superseeded. I
was unable to locate such a standard. If you know a more recent RFC that
obsoletes 2076, please let me know.

In section 3.9, Quality information, the overall comments for the Precedence
attribute are:

  non-standard
  controversial
  discouraged

The definition of these terms are:

   "non-standard"          This header is not specified in any of
                           referenced RFCs which define Internet
                           protocols, including Internet Standards, draft
                           standards or proposed standards. The header
                           appears here because it often appears in e-
                           mail or Usenet News. Usage of these headers is
                           not in general recommended. Some header
                           proposed in ongoing IETF standards development
                           work, but not yet accepted, are also marked in
                           this way.

   "discouraged"           This header, which is non-standard, is known
                           to create problems and should not be
                           generated. Handling of such headers in
                           incoming mail should be done with great
                           caution.

   "controversial"         The meaning and usage of this header is
                           controversial, i.e. different implementors
                           have chosen to implement the header in
                           different ways. Because of this, such headers
                           should be handled with caution and
                           understanding of the different possible
                           interpretations.

More specific comments for the Precedence attribute are:

   Sometimes used as a priority
   value which can influence
   transmission speed and delivery.
   Common values are "bulk" and
   "first-class". Other uses is to
   control automatic replies and to
   control return-of-content
   facilities, and to stop mailing
   list loops.

RFC 3834, Recommendations for Automatic Responses to Electronic Mail,

  http://tools.ietf.org/search/rfc3834

has these words to say about the Precedence field in section 3.1.8:

   A response MAY include a Precedence field [I4.RFC2076] in order to
   discourage responses from some kinds of responders which predate this
   specification.  The field-body of the Precedence field MAY consist of
   the text "junk", "list", "bulk", or other text deemed appropriate by
   the responder.  Because the Precedence field is non-standard and its
   interpretation varies widely, the use of Precedence is not
   specifically recommended by this specification, nor does this
   specification recommend any particular value for that field.


This posting:

  http://stackoverflow.com/questions/154718/precedence-header-in-email

suggests setting a null 'Return-Path' and to avoid 'Precedence'.

Google's Bulk Senders Guidelines

  https://support.google.com/mail/answer/81126?hl=en

suggests using 'Precedence: bulk' for such email. I'm not sure if roundup nosy
emails qualify as bulk emails.

The blog, Understanding the “Precedence:” Header

  http://blog.returnpath.com/blog/jd-falk/precedence

discusses this field, and states what may be a best practice.

I located who made this change to roundup. On 2008-07-21 Richard pushed a change
to roundup/mailer.py. The change was to add Precedence: bulk to the mail header
in the get_standard_message function. The comment on that change is "Send a
Precedence header in email so autoresponders don't". I sent a message to him on
sourceforge asking for rationale regarding adding this field.