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+# User Group Membership for Postfix
+
+Limit E-Mail access for users using group membership in Postfix.
+
+The default Postfix setup restricts E-Mail to all system users, which includes
+service users such as 'www'.
+
+## Installation
+
+```sh
+go build .
+sudo mv postfix-unix-users /usr/local/bin
+sudo chown root:root /usr/local/bin/postfix-unix-users
+```
+
+## Configuration
+
+Tell Postfix to start the program as part of its operation, using allowed user groups listed in the file '/etc/postfix/user_groups'.
+
+/etc/postfix/master.cf:
+```
+unix-users    unix  -       n       n       -       1       spawn
+  user=nobody argv=/usr/local/bin/postfix-unix-users
+    --hostname $myhostname
+    --groups /etc/postfix/user_groups
+    email-group-1 email-group-2
+```
+
+Tell Postfix to lookup local users using the program's unix socket.
+
+/etc/postfix/main.cf:
+```
+# Replace 'unix:passwd.byname' with 'socketmap:unix:private/unix-users:membership'.
+local_recipient_maps = socketmap:unix:private/unix-users:membership $alias_maps
+```