Regex-escape postfix_low_priority_domains

roles/mail/templates/postfix-out/transport_out.pcre

@@ -2,5 +2,5 @@
 # smtp transport.
 /(yahoo|hotmail|live|outlook)(\.[a-z]{2,3}){1,2}$/ smtp-lowpri:
 {% for pattern in postfix_low_priority_domains | default([]) %}
-/{{ pattern }}/ smtp-lowpri:
+/{{ pattern | regex_escape }}/ smtp-lowpri:
 {% endfor %}