diff --git a/docker-compose.public.yml b/docker-compose.public.yml
index 9a2c6c1..c0f41c6 100644
--- a/docker-compose.public.yml
+++ b/docker-compose.public.yml
@@ -21,6 +21,8 @@
 #
 # Mailbox admin@<domain>: its password is in the "mail" log on the first start and in /app/data/initial-passwords.txt.
 # Registration is closed: new mailboxes only by the admin. No catch-all.
+# Defaults are ours (mail.jambocheck.com / jambocheck.com) — Jambotron deploys without a .env; another domain:
+# MAIL_HOSTNAME, MAIL_DOMAINS, ACME_EMAIL in .env next to this file.
 name: reactive-mail-public
 
 services:
@@ -71,8 +73,8 @@ services:
           [ -s /tls/cert.pem ] && sleep 43200 || sleep 3600
         done
     environment:
-      MAIL_HOSTNAME: ${MAIL_HOSTNAME:?MAIL_HOSTNAME, e.g. mail.jambocheck.com}
-      ACME_EMAIL: ${ACME_EMAIL:-postmaster@${MAIL_DOMAINS}}
+      MAIL_HOSTNAME: ${MAIL_HOSTNAME:-mail.jambocheck.com}
+      ACME_EMAIL: ${ACME_EMAIL:-postmaster@jambocheck.com}
       LEGO_LOG_FORMAT: text
     volumes:
       - lego:/var/lib/lego
@@ -124,13 +126,13 @@ services:
     environment:
       DB_HOST: db
       MAILSERVER_SECRETS_DIR: /run/mail-secrets
-      MAIL_DOMAINS: ${MAIL_DOMAINS:?MAIL_DOMAINS, e.g. jambocheck.com}
-      MAIL_HOSTNAME: ${MAIL_HOSTNAME:?MAIL_HOSTNAME, e.g. mail.jambocheck.com}
+      MAIL_DOMAINS: ${MAIL_DOMAINS:-jambocheck.com}
+      MAIL_HOSTNAME: ${MAIL_HOSTNAME:-mail.jambocheck.com}
       MAIL_TLS_CERT: /tls/cert.pem
       MAIL_TLS_KEY: /tls/key.pem
       SMTPS_PORT: "3465"
       IMAPS_PORT: "3993"
-      DKIM_DOMAIN: ${MAIL_DOMAINS}
+      DKIM_DOMAIN: ${MAIL_DOMAINS:-jambocheck.com}
       DKIM_SELECTOR: mail
       DKIM_KEYS_DIR: /app/data/dkim-keys
       MAIL_CATCH_ALL: ""