Re-enable the built-in nginx resolver for traffic going through the mail plugin.

This is required for passing rDNS/ptr information to postfix.
The mail proxy uses the resolver info for passing XCLIENT info.
See http://nginx.org/en/docs/mail/ngx_mail_proxy_module.html#xclient
Without this info rspamd will flag all messages with DHFILTER_HOSTNAME_UNKNOWN due to the missing rDNS/ptr info.

core/nginx/conf/nginx.conf

@@ -277,6 +277,7 @@ 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;
     error_log /dev/stderr info;
 
     {% if TLS and not TLS_ERROR %}

towncrier/newsfragments/2368.bugfix

@@ -0,0 +1,3 @@
+Re-enable the built-in nginx resolver for traffic going through the mail plugin.
+This is required for passing rDNS/ptr information to postfix.
+Without this rspamd will flag all messages with DHFILTER_HOSTNAME_UNKNOWN due to the missing rDNS/ptr info.