[...]

This message was not delivered due to the following reason(s):

Your message was not delivered because the destination server was
unreachable within the allowed queue period. The amount of time
a message is queued before it is returned depends on local configura-
tion parameters.

Most likely there is a network problem that prevented delivery, but
it is also possible that the computer is turned off, or does not
have a mail system running right now.

Your message was not delivered within 6 days:
Mail server 42.112.15.186 is not responding.

The following recipients could not receive this message:
<lurker@lists.plug.phoenix.az.us>

Please reply to postmaster@ietf.org
if you feel this message to be in error.