[Postfix] Fix HELO name

data/Dockerfiles/postfix/postfix.sh

@@ -341,9 +341,11 @@ EOF
 
 sed -i '/User overrides/q' /opt/postfix/conf/main.cf
 echo >> /opt/postfix/conf/main.cf
-if [ -f /opt/postfix/conf/extra.cf ]; then
-  cat /opt/postfix/conf/extra.cf >> /opt/postfix/conf/main.cf
-fi
+touch /opt/postfix/conf/extra.cf
+sed -i '/myhostname/d' /opt/postfix/conf/extra.cf
+echo -e "myhostname = ${MAILCOW_HOSTNAME}\n$(cat /opt/postfix/conf/extra.cf)" > /opt/postfix/conf/extra.cf
+
+cat /opt/postfix/conf/extra.cf >> /opt/postfix/conf/main.cf
 
 if [ ! -f /opt/postfix/conf/custom_transport.pcre ]; then
   echo "Creating dummy custom_transport.pcre"