Francis Galiegue <fg AT one2team DOT com> writes:

> One problem is that not every mailing list management program agrees on what 
> headers should be defined that identify a message as being sent to a mailing 
> list. Some use X-Mailing-List, some others use Precedence, some others use 
> Sender (amanda-users is one of them), etc etc. Mailing list handling is not 
> enforced by an RFC.
>
> I've been asked to implement an automatic mail response system at my company 
> but refused to do so: I couldn't reliably identify bulk messages (mailing 
> lists) from others reliably. I subscribe to three different mailing lists, 
> each of which have a different idea of how to announce themselves as being 
> mailing lists. Heh.

It's pretty straighforward in RFC 3834: if the user's email address
actually appears in a To/CC/BCC header, then a response may be sent.
Otherwise not.  There's no need to determine if there is a mailing list
involved.