Guard against SMTP smuggling

This short-term workaround is recommended at https://www.postfix.org/smtp-smuggling.html:

    smtpd_data_restrictions=reject_unauth_pipelining

setup/mail-postfix.sh

@@ -69,6 +69,11 @@ tools/editconf.py /etc/postfix/main.cf \
 	maximal_queue_lifetime=2d \
 	bounce_queue_lifetime=1d
 
+# Guard against SMTP smuggling
+# This short-term workaround is recommended at https://www.postfix.org/smtp-smuggling.html
+tools/editconf.py /etc/postfix/main.cf \
+	smtpd_data_restrictions=reject_unauth_pipelining
+
 # ### Outgoing Mail
 
 # Enable the 'submission' ports 465 and 587 and tweak their settings.