2/7     I know I can use "dig" to find out what foo is MXed to, but is
        there a way to find out every MXed to bar ? (at least on an
        authoritative name server ?)
        \_ host -t MX -l <DEAD>your-domain.com<DEAD> | grep bar
        \_ Thats like trying to find out every webpage that links to a
           particular page.  There is no back reference, so you can't really
           know find this out.  Sure, looking at access logs might give you
           all the links that were used, but you'd still have an unknown
           number of unused ones out there.
        \_ No.  This is why relay-based-on-mx can be trivially spoofed.