Outlook for Mac - Unable to open this application

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