Michael P.:
I periodically check my mail server IP on http://multirbl.valli.org/ and/or http://www.dnsbl.info/ , to make sure it's not on any blocklist. (It was recently on one for cryptic reasons. When I politely inquired and stressed that I'd be glad to fix any problem but didn't understand the existing one, the listing was removed without comment.)
$ host lists.balug.org lists.balug.org has address 198.144.194.238 lists.balug.org has IPv6 address 2001:470:1f04:19e::2 lists.balug.org mail is handled by 100 mx.lists.balug.org. $
http://multirbl.valli.org/lookup/198.144.194.238.html shows that one cluster of blocklists, the rfc-clueless.org one, doesn't like your IP. This is the successor to Derek Balling's rfc-ignorant.org DNSBL, which Derek eventually shut down, and has the same mission. The listing policy is here: http://rfc-clueless.org/pages/listing_policy
Looks like they believe that 198.144.194.238 isn't accepting mail to postmaster. an RFC requirement for any FQDN that deals in SMTP (http://rfc-clueless.org/pages/listing_policy-postmaster).
When I did a quick check telneting to 25/tcp, it seems to me that the system _was_ going to accept my manually composed mail to postmaster@lists.balug.org -- so I'm unclear on why that listing's there.
Seems like they also have no-postmaster listings for FQDNs balug-sf-lug-v2.balug.org, balug.org, and temp.balug.org. Maybe you should actually cease accepting mail for those FQDNs.