sympl-mail: error codes returned by SpamHaus result in mails being blocked.
See https://forum.sympl.io/t/spamhaus-error-responses/378 for more details.
mail/sympl.d/10-acl/50-acl-check-rcpt/75-dns-blacklists
will need updating to change the lists from deny dnslists = <list>.spamhaus.org
to deny dnslists = <list>.spamhaus.org!&0.255.255.0
.
Edited by Paul Cammish