Anyone know why [log in to unmask] is having so many problems lately?

          Last error: Mailer mx-rr.home.com said: "550 5.7.1
                      <[log in to unmask]>... Relaying denied"

As list owner for several lists (on different LISTSERV
installations in different parts of the country), I'm receiving
quite a few delivery error reports like the one above, for a
number of [log in to unmask] subscribers on those several lists.

Peter R