I think I've figured it out. The key is here:

Jan 10 20:07:14 shelmerston postfix/smtpd[25950]: 7EC8D73CB9: reject:
RCPT from sv12pub.verizon.net[206.46.252.148]: 550
<owner-saulnachem*XXX**VERIZON*[log in to unmask]>: User
unknown in local recipient table;
from=<[log in to unmask]>
to=<owner-saulnachem*XXX**VERIZON*[log in to unmask]>
proto=SMTP helo=<sv12pub.verizon.net>

Looks like Verizon is using an anti-spam solution that actually tries 
to do an SMTP handshake with the sending server to check if the 
bounce address (aka the RFC821 MAIL FROM address aka the Return-Path: 
address) exists.

So the problem really IS that Postfix on your server is not 
configured to accept probe messages. This means that once the Postfix 
configuration is fixed, the Verizon problem should be too. Postfix is 
outside my area of expertise, so I don't have the final solution for 
you yet, but I think we have found a very likely candidate. 

The question to be answered now is:

How do you tell Postfix to forward all mail for HMSSURPRISE.ORG to 
LISTSERV and let LISTSERV decide how to deal with it?

-- 
Francoise Becker <[log in to unmask]>

Knowledge is just a click away: http://www.lsoft.com/optin.html