This Google article has some details and instructions on customizing spam filters:
https://support.google.com/a/answer/2368132?hl=en
Below is that article:
Create and use an approved sender list
You can create an approved sender list (based on email or an entire domain) to bypass any spam filters. This setting only applies to incoming messages. In addition, approved sender lists can be used in the following ways:
- Reuse approved sender lists across different settings. For example, you can specify the same list in the Spam setting and the Blocked senders setting to modify the behavior of both settings.
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Gmail checks the addresses or domain names that you enter against the From part of the message header, and not the Return-Path part of the message header. For this reason, From must match an address or domain you entered in the list.
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Match and add both domain and sub-domain names to the approved sender list, for example abc.com and xyz.abc.com.
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In some cases, a message that's blatantly spam, although sent from an approved sender, can still be marked spam and delivered to a recipient's spam folder.
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If a message from an approved sender contains a virus or is part of an email attack, Google's virus filters will prevent it from reaching your users.
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A message from an approved sender will still be blocked if directed to do so by the sender's Domain-based Message Authentication, Reporting, and Conformance (DMARC) policy.
Customize a spam filter
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Sign in to your Google Admin console.
Sign in using your administrator account (does not end in @gmail.com).
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From the Admin console Home page, go to Apps
G Suite
Tip: To see Advanced settings, scroll to the bottom of the Gmail page.
- (Optional) On the left, select an organization.
- Scroll to the Spam section, hover over the Spam setting, and click Configure. If the setting is already configured, hover over the setting and click Edit or Add Another.
- For a new setting, enter a unique name.
- 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.
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To bypass spam filters for messages from users in the same organization, check the Bypass spam filters for messages received from internal senders box.
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To bypass spam filters for messages from addresses or domains specified in an approved sender list:
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Check the Bypass spam filters for messages received from addresses or domains within these approved senders lists box.
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Click Use existing or create a new one, enter a new list name, and click Create.
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To use an existing list as your approved sender list, click the list name.
To add email addresses or domains to a list:
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Hover over the list name, click Edit, and then click Add.
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Enter an email address or domain name.
Important: Use the Do not require sender authentication option with caution as it can lead to spoofing. The option bypasses the spam folder for approved senders that don't have authentication, such as SPF or DKIM, enabled. -
Click Save.
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Repeat the steps to add more email addresses or domains to the list.
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- To send filtered messages to Admin Quarantine for review, check the Put spam in administrative quarantinebox. If you select this option, spam messages are never directed to users' spam folders. If you are using the Message Center, spam messages aren't directed to the Message Center. A message that's released from quarantine is delivered directly to the user's Gmail inbox or non-Gmail mailbox.
- Click Add Setting.
- At the bottom of the Gmail Advanced setting page, click Save.
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