Sender Policy Framework (SPF) is a security protocol for email communication. It provides that the domain of the email address specifies in a DNS record which servers are allowed to send emails on its behalf. The authorised servers are trusted by the receiving mail servers, and they accept emails from these servers as trustworthy.
When you use SPF, you increase trust in your email and reduce the likelihood that it will be marked as spam. SPF does an important job in curbing spoofing and spam, but for technical reasons, it cannot provide 100% protection.
If you send your emails with IONOS, use IONOS SPF records.