[Exim] generating 551 return codes

Users who leave the company are given the choice of having a forwarding
address for email returned as part of a failure message (rather than
accepting and forwarding mail for them)

i.e. the folowing alias is used
bob: :fail: Bob Smith is no longer at Example Ltd, try <bob@???>
to generate the following error
550 <bob@example> Bob Smith is no longer at Example Ltd, try <bob@???>

As I understand from RFC2821 this might be more useful as a 551 error code?
551 Bob Smith is no longer at Example Ltd, try <bob@???>

I can't see anyway to easily do this in 3.4 - will this be available in v4?