In the Automation section (System Preferences > Security & Privacy - Privacy tab), FlexiBee is listed and you should verify whether the Mail app is not listed twice underneath it. This causes the Mail app to open without opening a new email message window. If the FlexiBee app is not listed there at all, try simply reinstalling the app.

First, quit the Flexi app.

Launch the Terminal app and enter the following command:
​tccutil reset All eu.flexibee.client

​(You can find Terminal in Applications under the Utilities folder, or you can launch it via Spotlight by pressing the Command + Space keyboard shortcut. Then simply type Terminal into the Spotlight search field and confirm by pressing Enter.)

This command clears the Automation section so that the Mail app is assigned to FlexiBee only once.

If for some reason this command does not work (returns an error or fails to remove the entry from Automation), a more drastic action is required — clear the entire Automation section using the command:
​tccutil reset AppleEvents
​

WARNING: This command will remove all permissions for all other apps in the Automation section and you will need to grant them again.

Then restart your computer.

After launching Flexi and sending an email from the app, a prompt should appear asking whether you want to allow FlexiBee to access the Mail app. Grant access, and sending emails from the Mail app should then work without any issues.

If the above access permission prompt does not appear in Flexi, download and reinstall Flexi.