When a message is rejected by the spam kill level Amavis will normally respond with the reply "spam":
554 5.7.0 Reject, id=27440-12 - spam (in reply to end of DATA command)
If you want to change this to something more descriptive on Centos/RHEL/CloudLinux/AlmaLinux edit the file /etc/amavisd/warden.conf or on Debian/Ubuntu edit the file /etc/amavis/conf.d/99-warden and add the line:
$smtp_reason_by_ccat{&CC_SPAM} = 'id=%n - rejected for policy reasons';
After making these changes restart Amavis:
// Centos/RHEL/CloudLinux/AlmaLinux
systemctl restart amavisd
// Debian/Ubuntu
systemctl restart amavis