Silence some errors in nginx

"could not be resolved (3: Host not found) while in resolving client
address, client:"

core/nginx/conf/nginx.conf

@@ -278,7 +278,6 @@ 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;
     error_log /dev/stderr info;
 
     {% if TLS and not TLS_ERROR %}

towncrier/newsfragments/2346.bugfix

@@ -0,0 +1 @@
+Disable the built-in nginx resolver for traffic going through the mail plugin. This will silence errors about DNS resolution when the connecting host has no rDNS.