mail1.posten.se has no CNAME records.
Question and response
Question
dig mail1.posten.se. NS
Answer
The response has no answer records.
Authority
posten.se. 1964 SOA ns1.p201.dns.oraclecloud.net. registry.ports.net. 2026030609 1800 900 1814400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0