The StartDNSBL statement requires a hostname and DNSBL zone, in the format

StartDNSBL <hostname> <DNSBL zone>

The DNS blacklist lookup begins immediately, but the script is allowed to continue. When the DNSBL statement is reached, the script will get the result of the corresponding StartDNSBL statement. If the DNS blacklist lookup is still in progress, it will allow the lookup to complete, before returning the result.

StartDNSBL ${SenderIP} sbl.spamhaus.org

# Continue with the rest of the script, whilst also running the DNS blacklist lookup

# By the time the script reaches the DNSBL statement, the lookup should either be fully or partially completed.

DNSBL ${SenderIP} sbl.spamhaus.org dnsresult