spf-b.mymxserver.com has no CNAME records.
Question and response
Question
dig spf-b.mymxserver.com. NS
Answer
The response has no answer records.
Authority
mymxserver.com. 3514 SOA ns01.agenturserver.de. support.mittwald.de. 1701746689 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0