custom reject message in smtp-dialog

Hi,

usually when amavisd-new rejects mail as pre-queue filter, the 
smtp-dialog-message is like "554 5.7.0 Reject, id=13158-01 - spam"

I would like to customize the message, if the Spamassssin-Rule SPF_FAIL 
triggers, e.g.
"554 5.7.0 Reject, id=13158-01 - SPF Failure ; Please contact your 
administrator"

or on any other custom message on any other custom spamassassin-rule.

How can i achieve this?

amavis-new 2.11.0
spamassassin 3.4.2


Thanks in advance

Ralf