I too received Mail Undeliverable message today:
The original message was received at Mon, 14 Apr 2008
18:04:32 -0600
from web31702.mail.mud.yahoo.com [68.142.201.182]
----- The following addresses had permanent fatal
errors -----
marilyn@javaranch.com (reason: 554 IMTA18.emeryville.ca.mail.comcast.net
comcast 66.114.98.26 Comcast BL004 Blocked for spam.
Please see
http://www.comcast.net/help/faq/index.jsp?faq=SecurityMail_Policy18628) (expanded from: <
nitpick@javaranch.com>)
----- Transcript of session follows -----
... while talking to mx1.comcast.net.:
<<< 554 IMTA27.westchester.pa.mail.comcast.net comcast
66.114.98.26 Comcast BL004 Blocked for spam. Please
see
http://www.comcast.net/help/faq/index.jsp?faq=SecurityMail_Policy18628 ... while talking to mx2.comcast.net.:
<<< 554 IMTA18.emeryville.ca.mail.comcast.net comcast
66.114.98.26 Comcast BL004 Blocked for spam. Please see
http://www.comcast.net/help/faq/index.jsp?faq=SecurityMail_Policy18628 554 5.0.0 Service unavailable