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Sorry, we have no information about mail.billingagent.com. billingagent.com is delegated to two nameservers, however both delegated nameservers are missing in the zone. All nameservers are at worldnic.com. They are on different IP networks.

billingagent.com
ns records
ns21.worldnic.com
+ 205.178.190.11 205.178.160.0/19 AS6245 -
+ --- -
ns22.worldnic.com
+ 205.178.144.11 205.178.144.0/24 AS19871 -
nameservers missing in zone
ns21.worldnic.com
ns22.worldnic.com