Configure a resolver for the mail server to populate xclient hostnames

core/nginx/conf/nginx.conf

@@ -141,6 +141,7 @@ mail {
     server_name {{ HOSTNAMES.split(",")[0] }};
     auth_http http://127.0.0.1:8000/auth/email;
     proxy_pass_error_message on;
+    resolver {{ RESOLVER }} valid=30s;
 
     {% if TLS and not TLS_ERROR %}
     include /etc/nginx/tls.conf;