This version will allow the private server owner to show a different email then "@grasscutter.io" if they want.

src/main/java/emu/grasscutter/config/ConfigContainer.java

@@ -140,6 +140,7 @@ public class ConfigContainer {
         public boolean autoCreate = false;
         public boolean EXPERIMENTAL_RealPassword = false;
         public String[] defaultPermissions = {};
+        public String playeremail = "grasscutter.io";
         public int maxPlayer = -1;
     }
 

src/main/java/emu/grasscutter/game/Account.java

@@ -109,7 +109,7 @@ public class Account {
             return email;
         } else {
             // As of game version 3.5+, only the email is displayed to a user.
-            return this.getUsername() + "@grasscutter.io";
+            return this.getUsername() + "@"+ACCOUNT.playeremail;
         }
     }
 
@@ -235,7 +235,7 @@ public class Account {
             this.addPermission("*");
         }
 
-        // Set account default language as server default language
+        // Set account default language as fserver default language
         if (!document.containsKey("locale")) {
             this.locale = LANGUAGE;
         }