mailbox.promoexpress.com.ec has no CNAME records.
Question and response
Question
dig mailbox.promoexpress.com.ec. NS
Answer
The response has no answer records.
Authority
com.ec. 300 SOA master.nic.ec. admin.nic.ec. 2026040191 600 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0