spf.ext.tds.de has no CNAME records.
Question and response
Question
dig spf.ext.tds.de. NS
Answer
The response has no answer records.
Authority
tds.de. 136 SOA ns1.manage-now.de. security.tds.de. 2025120701 21600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0