The Sender Policy Framework (SPF) procedure makes it difficult to forge email sender addresses. The receiving mail server checks whether the sending mail server is authorised to send emails (for example, newsletters) with a specific sender domain. If the receiving mail server determines that the sending mail server is not allowed to send emails for a specific sender domain, the email can be classified as spam and placed in the recipient's spam folder. The delivery of emails is therefore not prevented due to a failed send authorisation. However, the sender’s reputation will be damaged and the newsletter is unlikely to be read.
Note: The SPF procedure only checks the domain of the sender address. To check the sender and email content, you must also use DKIM (DomainKeys Identified Mail) and DMARC (Domain-based Message Authentication, Reporting and Conformance).