added toString()

application/module-mail/src/main/java/org/fuseri/modulemail/service/EmailDto.java

@@ -12,4 +12,12 @@ public class EmailDto {
         this.content = content;
     }
 
+    @Override
+    public String toString() {
+        return "EmailDto{" +
+                "sender='" + sender + '\'' +
+                ", receiver='" + receiver + '\'' +
+                ", content='" + content + '\'' +
+                '}';
+    }
 }