Enabling end-user spam notification messages lets your users manage their own quarantined spam, bulk, and phishing messages.

End-user spam notifications contain a list of all spam-quarantined messages that the end user has received during a time period that you configure (you can specify a value between 1 and 15 days). You can also configure the language in which the notification message is written.

After receiving a notification message, end users can choose from the following options:

Block Sender if you want Office 365 to add the sender to your blocked senders list.

Release if the message isn't spam and you want Office 365 to send the message to your mailbox.

Review to navigate to the Quarantine Portal within the Security & Compliance Center if you want to take other actions, such as Preview or Release.

Use the EAC to configure end-user spam notifications

1. In the Exchange admin center (EAC), navigate to Protection > Spam filter.
2. Select the spam filter policy for which you want to enable end-user spam notifications (they are disabled by default).
3. In the right pane, where the summary information about your policy appears, click the Configure End-user spam notifications link.
4. In the subsequent dialog box, you can configure the following options:
   • Enable end-user spam notifications Select this check box in order to enable end-user spam notifications for this policy. (Conversely, if this policy is enabled, you can clear this check box in order to disable end-user spam notifications for this policy.)
   • Send end-user spam notifications every (days) Specify how often to send end-user spam notifications. The default is 3 days. You can specify between 1 and 15 days. If you specify 7 days, for example, the notification will include a list of all messages intended for that user within the past 7 days that were sent to the spam quarantine instead.
   • Notification language Using the drop-down list, select the language in which to write end-user spam notifications for this policy.
   • Click Save. A summary of your spam filter policy settings, including your end-user spam notification settings, appears in the right pane.

To test end-user spam notifications by sending them to a limited set of users before fully implementing them, create a custom spam filter policy that enables end-user spam notifications for the domains in which the users reside. Then, in the EAC, under Mail flow > rules, create a mail flow rule (also known as a transport rule) to block messages from quarantine@messaging.microsoft.com (the email address that sends notifications) with exceptions for the users who you want to receive the notifications. The following image is an example of creating an exception for two users (SaraD and AlexD) from domain Contoso.com:

Use the SCC to configure end-user spam notifications

You can also use the Security and Compliance Center (SCC) to configure end-user spam notifications. Follow these steps:

1. Open the Security and Compliance Center, navigate to Threat management > Policy > Anti-spam or use the direct link https://protection.office.com/antispam.
2. Click the down arrow next to the spam filter policy for which you want to enable end-user spam notifications.
3. Click on the Configure End-user spam notifications link.
4. In the subsequent dialog box, you can configure the following options:
   • Enable end-user spam notifications Select this check box in order to enable end-user spam notifications for this policy. (Conversely, if this policy is enabled, you can clear this check box in order to disable end-user spam notifications for this policy.)
   • Send end-user spam notifications every (days) Specify how often to send end-user spam notifications. The default is 3 days. You can specify between 1 and 15 days. If you specify 7 days, for example, the notification will include a list of all messages intended for that user within the past 7 days that were sent to the spam quarantine instead.
   • Notification language Using the drop-down list, select the language in which to write end-user spam notifications for this policy.
   • Click Save. A summary of your spam filter policy settings, including your end-user spam notification settings, appears in the pane.

Source: https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/configure-end-user-spam-notifications-in-exchange-online