Is this issue is coming for all users or specific user. 

First Check - In Recipient User record , Notification is enabled or not.

If it is enabled , then check following:

Platform Configuration: Hopefully environment is production as in non-prod we disable the notification for users.Check all outbound and inbound email properties are correctly configured or not.

 Notification Devices :Confirmed that the user's primary email address is listed, active, and correct.

Email Address Accuracy:Double-checked to ensure the email address is valid and correctly configured.

Notification Subscription :Verify that the user is subscribed to the appropriate notifications.

                                        The user record should  exist in the "cmn_notif_device" table and active

Email Client Filtering: Confirm with the user that there are no rules or filters set up in their email box that could redirect or block ServiceNow emails. Also checked spam/junk folders.

私のところでも同様のトラブルがありました。

Nowサポートのページのヘッダにもトラブルのバナーが出ていましたが、P2等のアラートメールは無しでした。

復旧済でメールも順次送信されている模様です。（メールログ見る限りです。）

@Taro21 

see is email sending is disabled, if yes then enable it

also check this link

Troubleshooting Outbound Email 

💡 If my response helped, please mark it as correct ✅ and close the thread 🔒— this helps future readers find the solution faster! 🙏

皆様

たくさんのコメントありがとうございます。

当方でもメール送信の障害であることが判明し、現在は復旧しています。