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Sorry, we have no information about sparrow-dns.pinboard.com. pinboard.com is a domain controlled by two nameservers at pinboard.com themselves. They are on different IP networks. Incoming mail for pinboard.com is handled by four mailservers. All of them are on the same IP network.

pinboard.com
ns-soa records
dns1.pinboard.com
+ 213.239.197.143 213.239.192.0/18 AS24940 falk.pinboard.com
+ --- falk.pinboard.com
ns records
dns1.pinboard.com
+ 213.239.197.143 213.239.192.0/18 AS24940 falk.pinboard.com
+ --- falk.pinboard.com
dns2.pinboard.com
+ 188.40.111.120 188.40.0.0/16 AS24940  
+ ---  
mx records
pinboard.com.s200a1.psmtp.com
+ 207.126.147.10 207.126.147.0/24 AS26910 s200a1.psmtp.com
+ --- s200a1.psmtp.com
pinboard.com.s200a2.psmtp.com
+ 207.126.147.12 207.126.147.0/24 AS26910 s200a2.psmtp.com
+ --- s200a2.psmtp.com
pinboard.com.s200b1.psmtp.com
+ 207.126.147.13 207.126.147.0/24 AS26910 s200b1.psmtp.com
+ --- s200b1.psmtp.com
pinboard.com.s200b2.psmtp.com
+ 207.126.147.14 207.126.147.0/24 AS26910 s200b2.psmtp.com
+ --- s200b2.psmtp.com