[exim] Not frozing on unexistent user

Host A relays mail to host B. A receives mail from outside the
network. When A accepts a mail for some unexistent recipient in B, the
mail is frozen on A.

Given that the condition of "unexistent" will remain on B, how can I
say A that generate a bounce when B reject the message, instead of
frozing it.

Regards,
maykel