refactor: reuse isDefaultFromAddress

backend/src/email/middlewares/email.middleware.ts

@@ -223,17 +223,10 @@ export const InitEmailMiddleware = (
     const { fromName: defaultFromName, fromAddress: defaultFromAddress } =
       parseFromAddress(config.get('mailFrom'))
 
-    // As part of a PSD directive, we have changed the defaultFromAddress to [email protected].
-    // To prevent any breaking changes, we must now support both the new and old default address
-    const allowedDefaultAddresses = [
-      defaultFromAddress,
-      '[email protected]',
-    ]
-
     if (
       //  user enters an email that is neither their own nor info@mail.postman.gov.sg
       fromAddress !== userEmail &&
-      !allowedDefaultAddresses.includes(fromAddress)
+      !isDefaultFromAddress(from)
     ) {
       logger.error({
         message: INVALID_FROM_ADDRESS_ERROR_MESSAGE,