We have taken and sanitized a log extract, which was a live example tracking
down a specific user's ('paulhill')
asserted inability to send email. We sanitized the userid, source IP
and host name and recipient userids, to appear to be
from our example server outsmtp.example.com ;
the range 172.16 ... 31.x.x is reserved in RFC 1918 as one of
the dedicated
private IP space ranges. That RFC states that such IP ranges are
recommended to be dropped at any border
router which does not 'know' it is in local use.