I FOUND THIS POST when I go through the Draft post in the backend of my blog. It was drafted last year when I moved some emails to the IONOS Email system. It is still helpful for the “Email Basic” users.
Here is a sample of one of my domains, whose email is served on the ionos.ca email service.
The domain is treated as an external domain in ionos.ca.
for example, the domain name is webexample.win
Add a txt record for this domain at the Cloudflare panel.
The content is v=spf1 include:_spf.perfora.net include:_spf.kundenserver.de ~all
If you want to know why use this string, go to the ionos help page.
Then add another txt record for DMARC
The content is v=DMARC1;p=quarantine;pct=10;ruf=mailto:email@example.com
For more details about this setting, go to help.