"Send REST API notifications by e-mail" is an option that you can configure to send an e-mail notification after users perform some actions on kintone using REST API.

The setting can be configured under the "E-mail Notifications" section on the "Feature Activation" screen.
Activating features
Configuring the E-mail Notifications setting

Notifications for REST API actions include the following notifications:

Example:

• A General notification sent when a record is added through REST API
• A notification sent when an assignee is updated through REST API
• A Per Record notification sent when a record is updated through REST API

Note that Reminder notifications are not triggered by API actions. Reminder notification e-mails will be sent regardless of the REST API notification setting. For example, if you disable Send REST API notifications by e-mail and then use REST API to add records, a Reminder notification e-mail is still sent when the reminder condition is met.

If you use REST API to automate actions on kintone and also enable "Send REST API notifications by e-mail", you may receive a large number of notifications by e-mail. To avoid receiving numerous e-mail notifications, disable the "Send REST API notifications by e-mail" option and check the notifications on kintone.
Checking Notifications