The Mail Merge add-on can use CSV files generated by other programs.

LibreOffice Writer (the wordprocessing program) has its own Mail Merge facility which can automatically generate and send emails to addresses in your database. Instructions are given here: https://help.libreoffice.org/Writer/Mail_Merge_Wizard If you search for 'Libre Office mail merge' online you will find several user-friendly tutorials; the Libre Office help forum is also an excellent resource. I have never done an email mail merge personally so I can't speak from experience, though I have used LibreOffice mail merge to produce paper documents. The solution suggested by Zenos -- to use the Thunderbird mail merge add-on -- would also work, but it would require you to export the addresses from your database as a .csv file and then import that into the add-on.

Yes to Suzanna! I've been doing email for something approaching 20 years, and early-on created contact databases for use in mail-merge. I see no reason why in all these years I've never seen any email app utilize anything other than their very own "address book". Because any standard dbase-type file might be used for many things, it's plain ignorant to fill in the same items over and over again.

Seems all that might be needed would be a link built-in to the email app which then uses some other Real database to be accessed as/for the information. That same database could well be located on some server which many users or employees could access. Limitations could be built into the database to give some protection to everyone.

A part of my life has been building / rehabbing computers. That means setting up Thunderbird and other etc apps time after time, and that really "gets old". Computer and software companies can and should be taking care of such stuff, rather than just producing more-of-the-same and keeping their "tricks" secret.