Revert to port 2525 without authentication for internal mail relay

The STARTTLS port 2587 requires TLS certificate verification, but
Cloudron's internal mail relay uses a wildcard cert for *.due.ren
which doesn't match the hostname 'mail'. Port 2525 is the internal
plain SMTP relay that doesn't require authentication or TLS for
connections from within the same container network.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

start.sh

@@ -123,10 +123,10 @@ JWT_SECRET_FILE="$SECRETS_DIR/jwt_secret"
 SESSION_SECRET_FILE="$SECRETS_DIR/session_secret"
 
 SMTP_HOST="${CLOUDRON_MAIL_SMTP_SERVER:-mail}"
-SMTP_PORT="${CLOUDRON_MAIL_STARTTLS_PORT:-2587}"
+SMTP_PORT="${CLOUDRON_MAIL_SMTP_PORT:-2525}"
 SMTP_ENCRYPTION=""
-SMTP_USERNAME="${CLOUDRON_MAIL_SMTP_USERNAME:-}"
-SMTP_PASSWORD="${CLOUDRON_MAIL_SMTP_PASSWORD:-}"
+SMTP_USERNAME=""
+SMTP_PASSWORD=""
 SMTP_EMAIL="${CLOUDRON_MAIL_FROM:-no-reply@$RP_ID}"
 SMTP_SENDER_NAME="${CLOUDRON_MAIL_FROM_DISPLAY_NAME:-Ente}"