zone transfer 'mydomain.com/IN' denied

[GeneralX]

I'm using v9.21, and noticed the below error repeats about 3-4 times every
hour.  Is this something I should be concern with?  Noticed the client IP
address are sprintlink's IP:

    ns2-auth.sprintlink.net
    ns3-auth.sprintlink.net
    ns1-auth.sprintlink.net

Sep 19 23:43:55 ns1 named[73]: client 144.228.254.10#63690: zone transfer
'mydomain.com/IN' denied
Sep 19 23:53:37 ns1 named[73]: client 144.228.255.10#45584: zone transfer
'mydomain.com/IN' denied
Sep 20 00:00:02 ns1 named[73]: client 206.228.179.10#33155: zone transfer
'mydomain.com/IN' denied