fix: move the mailserver dns config to the proper file

applications/email.nix

@@ -268,6 +268,14 @@ in {
 
     # set up dns record for it
     services.skynet.dns.records = [
+      # core record
+      {
+        record = "@";
+        r_type = "MX";
+        # the number is the priority in teh case of multiple mailservers
+        value = "10 mail.${cfg.domain}.";
+      }
+
       # basic one
       {
         record = "mail";