Looks like I need to use:badrcptto (qpsmtpd plugin)
So I need to edit
/var/service/qpsmtpd/config/badrcptto
Which is templated.
However the contents and anything else I can find does not help me with the format.
It contains:
<usual templage header>
! Sorry, bang paths not accepted here
@.*@ Sorry, multiple at signs not accepted here
^@ Invalid recipient address
% Sorry, percent hack not accepted here
# Format is pattern\s+Response - don't forget to tie the pattern if required