scripts: improve custom user-supplied Postfix configuration

docs/content/config/advanced/override-defaults/postfix.md

@@ -2,33 +2,34 @@
 title: 'Override the Default Configs | Postfix'
 ---
 
-The Postfix default configuration can easily be extended by providing a `docker-data/dms/config/postfix-main.cf` in postfix format.
-This can also be used to add configuration that is not in our default configuration.
+[Our default Postfix configuration](https://github.com/docker-mailserver/docker-mailserver/blob/master/target/postfix/main.cf) can easily be extended to add parameters or modify existing ones by providing a `docker-data/dms/config/postfix-main.cf`. This file uses the same format as Postfix `main.cf` does ([See official docs](http://www.postfix.org/postconf.5.html) for all parameters and syntax rules).
 
-For example, one common use of this file is for increasing the default maximum message size:
+!!! example "Example"
 
-```cf
-# increase maximum message size
-message_size_limit = 52428800
-```
+    One can easily increase the [backwards-compatibility level](http://www.postfix.org/postconf.5.html#compatibility_level) and set new Postscreen options:
 
-That specific example is now supported and can be handled by setting `POSTFIX_MESSAGE_SIZE_LIMIT`.
+    ```cf
+    # increase the compatibility level from 2 (default) to 3
+    compatibility_level = 3
+    # set a threshold value for Spam detection
+    postscreen_dnsbl_threshold = 4
+    ```
 
-!!! note
 
-    [Postfix documentation](http://www.postfix.org/documentation.html) remains the best place to find configuration options.
+!!! help "How are your changes applied?"
 
-Each line in the provided file will be loaded into postfix.
+    The custom configuration you supply is appended to the default configuration located at `/etc/postfix/main.cf`, and then `postconf -nf` is run to remove earlier duplicate entries that have since been replaced. This happens early during container startup before Postfix is started.
 
-In the same way it is possible to add a custom `docker-data/dms/config/postfix-master.cf` file that will override the standard `master.cf`. Each line in the file will be passed to `postconf -P`. The expected format is `<service_name>/<type>/<parameter>`, for example:
+---
+
+Similarly, it is possible to add a custom `docker-data/dms/config/postfix-master.cf` file that will override the standard `master.cf`. **Note**: Each line in this file will be passed to `postconf -P`, i.e. **the file is not appended as a whole** to `/etc/postfix/master.cf` like `docker-data/dms/config/postfix-main.cf`! The expected format is `<service_name>/<type>/<parameter>`, for example:
 
 ```cf
+# adjust the submission "reject_unlisted_recipient" option
 submission/inet/smtpd_reject_unlisted_recipient=no
 ```
 
-Run `postconf -P` in the container without arguments to see the active master options.
-
-!!! note
+!!! attention
     There should be no space between the parameter and the value.
 
-Have a look at the code for more information.
+Run `postconf -Mf` in the container without arguments to see the active master options.

target/scripts/startup/setup-stack.sh

@@ -790,20 +790,16 @@ function _setup_postfix_virtual_transport
 
 function _setup_postfix_override_configuration
 {
-  _log 'trace' 'Setting up Postfix Override configuration'
+  _log 'debug' 'Overriding / adjusting Postfix configuration with user-supplied values'
 
   if [[ -f /tmp/docker-mailserver/postfix-main.cf ]]
   then
-    while read -r LINE
-    do
-      # all valid postfix options start with a lower case letter
-      # http://www.postfix.org/postconf.5.html
-      if [[ ${LINE} =~ ^[a-z] ]]
-      then
-        postconf -e "${LINE}"
-      fi
-    done < /tmp/docker-mailserver/postfix-main.cf
-    _log 'trace' "Loaded '/tmp/docker-mailserver/postfix-main.cf'"
+    cat /tmp/docker-mailserver/postfix-main.cf >>/etc/postfix/main.cf
+    sleep 1
+    # do not directly output to 'main.cf' as this causes a read-write-conflict
+    postconf -n >/tmp/postfix-main-new.cf 2>/dev/null
+    mv /tmp/postfix-main-new.cf /etc/postfix/main.cf
+    _log 'trace' "Adjusted '/etc/postfix/main.cf' according to '/tmp/docker-mailserver/postfix-main.cf'"
   else
     _log 'trace' "No extra Postfix settings loaded because optional '/tmp/docker-mailserver/postfix-main.cf' was not provided"
   fi
@@ -817,13 +813,10 @@ function _setup_postfix_override_configuration
         postconf -P "${LINE}"
       fi
     done < /tmp/docker-mailserver/postfix-master.cf
-    _log 'trace' "Loaded '/tmp/docker-mailserver/postfix-master.cf'"
+    _log 'trace' "Adjusted '/etc/postfix/master.cf' according to '/tmp/docker-mailserver/postfix-master.cf'"
   else
     _log 'trace' "No extra Postfix settings loaded because optional '/tmp/docker-mailserver/postfix-master.cf' was not provided"
   fi
-
-  _log 'trace' "Set Postfix's compatibility level to 2"
-  postconf compatibility_level=2
 }
 
 function _setup_postfix_sasl_password