mail.servidoraweb.net has no CNAME records.
Question and response
Question
dig mail.servidoraweb.net. NS
Answer
The response has no answer records.
Authority
servidoraweb.net. 9639 SOA dns1.servidoraweb.net. servers.servidoraweb.com. 2026020401 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0