If you try and setup mail merge with Apple Mail in LO Preferences the normal way it will either give you an error message or hang requiring a force quit and restart of LO. This is due to the Apple Security Protocols. To properly set up LO with Apple Mail you need to first go to this website:
Apple Manage ID’s
Then log in with you Apple ID and Password (email address & PSW) and click on “App-Specific Passwords”. Then click on the plus sign (+) to the right of the word “Passwords” and set up an Apple specific password for LibreOffice. Copy it is and save it somewhere as once you close the window you will not be able to see it again but will have to setup a new one as Apple hides it forever. Then use the APP password you have created for Apple Mail in LO to set up the SMTP server in LO Writer preferences. If this seems unfriendly and complicated, it is not LO’s fault as Apple no longer makes such things easy or user friendly.