mx01.dsl4u.ca has no CNAME records.
Question and response
Question
dig mx01.dsl4u.ca. NS
Answer
The response has no answer records.
Authority
dsl4u.ca. 10377 SOA auth01.dns.acceleratedconnections.com. hostmaster.dsl4u.ca. 2025072300 14400 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0