Fix: use static IP for mailman-core and update documentation

- Assign static IP 172.29.199.10 to mailman-core to ensure host Postfix can reach LMTP.
- Update MM_HOSTNAME and mailman-extra.cfg to use the static IP.
- Update README.md with correct networking recommendations for host-based MTAs.
- Add missing configuration files to repository.

conf/mailman-core/mailman-extra.cfg

@@ -0,0 +1,36 @@
+[mailman]
+# This address is the "site owner" address.  Certain messages which must be
+# delivered to a human, but which can't be delivered to a list owner (e.g. a
+# bounce from a list owner), will be sent to this address.  It should point to
+# a human.
+site_owner: scott.idem+admin@oneskyit.com
+
+
+[mta]
+incoming: mailman.mta.postfix.LMTP
+outgoing: mailman.mta.deliver.deliver
+# I am pretty sure lmtp_host should *not* be the same as the $MM_HOSTNAME! It should be the hostname of the Postfix server running the MTA.
+# The lmtp_host value is used exactly as is. It does not seem to replace the environment variable.
+# lmtp_host:
+# lmtp_host: $MM_HOSTNAME
+# lmtp_host: localhost
+# lmtp_host: mailman-core
+# lmtp_host: dgr-srv-prime
+# lmtp_host: 127.0.0.1
+# lmtp_host: 104.237.143.4
+# lmtp_host: 172.29.199.1 # No
+lmtp_host: 172.29.199.10
+lmtp_port: 8024
+# smtp_host: 127.0.0.1
+smtp_host: 104.237.143.4
+# smtp_port: 25
+# smtp_port: 465
+smtp_port: 587
+smtp_use_tls: True
+# smtp_host_user: scott
+# smtp_host_password: Adapting.7df
+smtp_secure_mode: starttls
+smtp_verify_hostname: False
+smtp_verify_cert: False
+
+configuration: /etc/postfix-mailman.cfg