[Exim] Dealing with 451 errors

Hello,

A user on site 'A' sends an email to site 'B'. Site 'B'
forwards the message automatically over to site 'C'.

Site 'C' (running Exim) attempts to verify the sender's
address and it fails, so it issues a 451 error code.

Now, what should site 'B' do in response to this?

Should it sit on the message for a while, periodically
retrying it? Or should it immediately fail the message
and bounce it back to the original sender as a permanent
error? What would Exim do if it was running on site 'B'?

Mark