Not having SPF is a clear indication you have little idea of what you're doing with an email server. A properly configured email server would have both SPF and

Not having SPF is a clear indication you have little idea of what you're doing with an email server. A properly configured email server would have both SPF and DKIM signing. Google's spam filter is a complicated black box. You're not going to get far understanding it by prodding it. But you're certainly not going to pass it by ignoring SPF.