FYI, our university-based listserver somehow made it on a list of
spamservers recently.  Many ISP's then simply refused any of our mail.  The
error message reads as follows:

Final-Recipient: RFC822; <medphys subscriber's email address removed>
Action: failed
Status: 5.1.3
Remote-MTA: DNS; nymx01.<domain>
Diagnostic-Code: SMTP; 553 5.3.0 Mail from 141.217.1.57 rejected;see
http://mail-abuse.org/cgi-bin/nph-rss? 141.217.1.57
Last-Attempt-Date: Thu, 8 Mar 2001 18:45:41 -0500 (EST)

If you know your server's IP address, you can check the website mentioned to
see if this is why you may be having delivery problems.