How to protect a server against mail spoofing?
Enable SPF check for incoming emails:
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).
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.
How to protect Plesk mail server from incoming spam