[Mail] Revert e0260299, make Postfix listen to port 25

Otherwise we won't receive emails not submitted on port 587

pica-mail/docker-compose.yml

@@ -40,6 +40,7 @@ services:
     build: ./pica-mail-mta
     container_name: pica-mail-mta
     ports:
+      - "25:25"
       - "587:587"
     networks:
       - mail