Update firststeps-local_mta.md

docs/firststeps-local_mta.md

@@ -11,8 +11,12 @@ Furthermore, to relay over a dockerized mailcow, you may want to add `172.22.1.1
 postconf -e 'relayhost = 172.22.1.1'
 postconf -e "mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128"
 postconf -e "inet_interfaces = loopback-only"
+postconf -e "relay_transport = relay"
+postconf -e "default_transport = smtp"
 ```
 
+**Now it is important** to not have the same FQDN in `myhostname` as you use for your dockerized mailcow. Check your local (non-Docker) Postfix' main.cf for `myhostname` and set it to something different, for example `local.my.fqdn.tld`.
+
 "172.22.1.1" is the mailcow created network gateway in Docker.
 Relaying over this interface is necessary (instead of - for example - relaying directly over ${MAILCOW_HOSTNAME}) to relay over a known internal network.