Your domain does not say, or not strictly enough, which servers may send mail on its behalf. Anyone can then send mail that seems to come from you.
A fraudster can send a fake invoice or a phishing mail with your domain as sender. Your clients trust it because it looks like your address. Without SPF your real mail also lands in spam more often.
This is part of the security check, a separate check next to the crawl, in Studio and Bureau, and only on a site you have verified. Snaglist only looks at what any visitor can see from outside. It never logs in, guesses no passwords and sends no attacks. It reads the SPF record of the domain from the DNS, through Quad9 in Switzerland, and reports it when it is missing or lets every server send.
In the report for your client this appears under the heading "Anyone can send mail on behalf of your domain". When everything is in order, the report says: "The domain says which servers may send its mail (SPF)".
Snaglist checks every page of your site, for this issue and all the others. Start for free, no credit card needed.
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