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Check the SPF (Sender Policy Framework) record for t-online.de. SPF specifies which mail servers are authorized to send email on behalf of this domain.
SPF (Sender Policy Framework) is a DNS TXT record published at t-online.de that lists the IP addresses and servers authorized to send email on behalf of the domain. When a receiving mail server gets an email claiming to be from t-online.de, it checks the SPF record to verify the sending server is authorized. Without a valid SPF record, emails from t-online.de are more likely to be marked as spam or rejected entirely. SPF is one of the three pillars of email authentication, alongside DKIM and DMARC.