When you forward an email to Fax.Plus for faxing to your intended recipient and you receive a response from Fax.Plus with the message: "SPF check failed" or "SPF check for your domain returned no results," this indicates that your DNS server lacks a proper SPF record.
Benefit: For security considerations, we exclusively allow emails from domains that have established a valid SPF record in their DNS settings.
You can check your SPF record here: https://app.dmarcanalyzer.com/dns/spf.
To add an SPF record to your DNS you should contact your account Admin, or if you are the admin of your DNS, you can find information on the web on how SPF records are set. Once you have set your SPF records, your emails should go through without an issue.