If the message "Unable to open BoostMyMailRelease.pkg because this app comes from an unidentified developer" appears when you’re installing Boost My Mail, follow the procedure below
- Go to System Preferences,
- Click on Security and privacy,
- Select the General tab,
- Click on the lock and enter your password in order to make changes,
- Select App Store and identified developers.
This is an Apple procedure: https://support.apple.com/fr-fr/HT202491