mail.mail1.tuthost.com has no NS records.
NS records for this zone are defined at mail1.tuthost.com.
NS records for this zone are defined at mail1.tuthost.com.
Question and response
Question
dig mail.mail1.tuthost.com. NS
Answer
The response has no answer records.
Authority
mail1.tuthost.com. 14400 SOA ns1.tuthost.com. hostmaster.tuthost.com.mail1.tuthost.com. 2017053110 14400 3600 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0