[exim] 421 vs 451

Hello,

I noticed that several large ISPs are denying service (closing an
SMTP-connection at "connect") if someone is listed in a rbl with the
code 421 (<domain> Service not available, closing transmission channel),
while "defer" in an ACL in exim causes 451 to be send (Requested action
aborted: local error in processing). is there a way to send 421 as 451
implies that something locally on our side is wrong, which is not the fact?

Regards
Marten