diff -urNp -x '*.orig' sendmail-8.16.1.org/cf/feature/redirect.m4 sendmail-8.16.1/cf/feature/redirect.m4
--- sendmail-8.16.1.org/cf/feature/redirect.m4	2020-05-19 21:54:31.000000000 +0200
+++ sendmail-8.16.1/cf/feature/redirect.m4	2021-10-03 22:30:57.858924594 +0200
@@ -18,6 +18,7 @@ divert(-1)
 
 LOCAL_RULE_0
 # addresses sent to foo@host.REDIRECT will give a 551 error code
+R$*@$*@$*			$#error $@ 5.7.1 $: "551 What is your problem, Willy?"
 R$* < @ $+ .REDIRECT. >		$: $1 < @ $2 . REDIRECT . > < ${opMode} >
 R$* < @ $+ .REDIRECT. > <i>	$: $1 < @ $2 . REDIRECT. >
 R$* < @ $+ .REDIRECT. > < $- >	$#error $@ 5.1.1 $: "551 User has moved; please try " <$1@$2>