When you give the SPF Tool an email or a domain it does not test the SPF record, but rather tests if an IP address is an ok sender based on the SPF record.

When you do not provide an IP to test in the tool's "optional server" box,  the sending ip is dnsstuff.com, thus SOFTFAIL you will see since OUR server tried to send mail on behalf of your domain.

If you are attempting to use this tool to validate your mail server against your SPF record, then you would need to submit that ip in the box under the domain box to perform the test.

Our Mail Server Test Center and DNSReport Tool results in regards to pass or fail are based on the existance of an SPF, where as this test (when provided a domain) evalulates whether a mail server is authorized to send mail on behalf of a domain via the SPF.