imap.mail1.tuthost.com has no CNAME records.
Question and response
Question
dig imap.mail1.tuthost.com. NS
Answer
The response has no answer records.
Authority
mail1.tuthost.com. 5206 SOA ns1.tuthost.com. hostmaster.tuthost.com.mail1.tuthost.com. 2017053110 14400 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0