In case anyone else runs into this, we:
1. Disabled RHSBL using:
config setprop qpsmtpd RHSBL disabled
signal-event email-update
2. Updated the DNSBL servers using:
config setprop qpsmtpd RBLList 'zen.spamhaus.org:b.barracudacentral.org:bl.spamcop.net'
signal-event email-update
When convenient we will re-enable RHSBL with new servers:
config setprop qpsmtpd SBLList 'badconf.rhsbl.sorbs.net:nomail.rhsbl.sorbs.net'
config setprop qpsmtpd RHSBL enabled
signal-event email-update
Note: the above changes were based largely on a verbal exchange, on the fact that they were easy to try, and on the fact that they worked.