Sometimes you may duplicate your production Matomo environment in a test or staging server. When you do so, your staging/testing Matomo instance may end up sending scheduled email reports or custom alerts to the people who had setup email reports in the production instance. You can prevent any emails from being sent from your test instance by adding the following setting under the [General] section in your config/config.ini.php:

[General]
; do not send any emails to users (including password reset, scheduled email reports, custom alerts, etc.)
emails_enabled = 0