Add hint for local MTA and port blocking issues

docs/first_steps.md

@@ -136,6 +136,16 @@ server {
 }
 ```
 
+## Install a local MTA
+
+The easiest option would be to disable the listener on port 25/tcp.
+
+**Postfix** users disable the listener by commenting the following line (starting with `smtp` or `25`) in `/etc/postfix/master.cf`:
+```
+#smtp      inet  n       -       -       -       -       smtpd
+```
+Restart Postfix after applying your changes.
+
 ## Sender and receiver model
 
 When a mailbox is created, a user is allowed to send mail from and receive mail for his own mailbox address.

docs/install.md

@@ -35,6 +35,8 @@ nano mailcow.conf
 ```
 If you plan to use a reverse proxy, you can, for example, bind HTTPS to 127.0.0.1 on port 8443 and HTTP to 127.0.0.1 on port 8080.
 
+You may need to stop an existing pre-installed MTA which blocks port 25/tcp. See [this chapter](https://andryyy.github.io/mailcow-dockerized/first_steps/#install-a-local-mta) to learn how to reconfigure Postfix to run besides mailcow after a successful installation.
+
 5\. Pull the images and run the composer file. The paramter `-d` will start mailcow: dockerized detached:
 ```
 docker-compose pull