Question
How to protect a server against mail spoofing?
Answer
Enable SPF check for incoming emails:
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In Plesk for Linux: Log in to Plesk > Tools & Settings > Mail Server Settings > SPF spam protection > Enable SPF spam protection to check incoming mail. Set SPF checking mode to Reject mail when SPF resolves to "fail"(deny).
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In Plesk for Windows: It is required to configure the Mail Server manually.
Additionally, it is recommended to enable DKIM and DMARC anti-spam solutions: What is DKIM, SPF and DMARC protection?
More information can be found in Official Plesk documentation.