However, I don't want spam checks for the bcc-user email to be bypassed. I
also don't want the spam that would be quarantined for the regular sender
to be delivered for the bcc-user either.
In other words, I want the bcc-user to be treated exactly the same as mail
for the intended recipient. Is this possible, given they are in different
domains?
I claim that this is not possible in a Postfix-always_bcc-before-Amavis
setup. Why don't you move always_bcc into the Postfix-service which
Amavis forwards to (usually 10025)?