Aggressive Spam Filtering

From my experience as Google Cloud Support agent, it's totally excessive to block every email that hasn't passed an SPF check. Even if it's technically possible to set up such a rule, you will end blocking emails you don't really want to block, as not every sender has a sanitized SPF record (even if it should be implemented by every responsible email admin).

However, if you're really worried about the customer receiving spam/phishing emails, you can set up the Spam setting to do Aggressive filtering, which will be more strict evaluating emails whenever SPF fails or suspicious keywords are found.

To set up aggressive spam filtering you can follow these steps: 

1) Access admin.google.com

2) Click on Apps  >  GSuite > Gmail > Advanced settings

3) Locate the Spam section and click on Configure

4) Enter a name for the rule

5) To configure more aggressive spam filtering, check the Be more aggressive when filtering spam box. If you select this option, it's likely that more messages will be marked spam and sent to your users' spam folders.

6) Click on Add setting and confirm with Save

Bear in mind it might take up to one hour for this change to propagate. You can get more information about Spam setting in the following article of our Help Center https://support.google.com/a/answer/2368132?hl=en.

Please, test this kind of filtering previously, as it might be too strict for the majority of customers.