Is this a cache poisoning attack?

[George Mitchell]

I'm getting a plethora of errors like this in my system log, as my
sendmail is attempting to validate the domain names of incoming
envelope senders of spammers (identifying info deleted):

DNS format error from xxx#53 resolving xxx/MX for client xxx:xxx: Name .
(SOA) not subdomain of zone xxx -- invalid response

FreeBSD 10.3-RELEASE-p18, BIND 9.11.0-P5.  Name service seems to be
working normally otherwise.  Is this a symptom of an attempted cache
poisoning attack?                                          -- George

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