Neal Becker gives an example:
[...]
> 1997-03-14 00:46:45 0w5PoV-0002Kb-00 rejected from mtigwc03.worldnet.att.net [204.127.131.3]: temporarily unable to verify sender address (try again later) <ExchangeCurrency@???>
Sorry to see you are suffering from these spammers as well! There's also
spam with with null envelope sender coming from the same source... [Philip
has pointed out that mailhost.worldnet.att.net is an ISP who are being used
as a spam relay: they claim to be trying to get relay blocks into place.]
Chris Thompson Cambridge University Computing Service,
Email: cet1@??? New Museums Site, Cambridge CB2 3QG,
Phone: +44 1223 334715 United Kingdom.
