I have problems in list of contacts of Pantheon-mail. I found that it is not possible to manage this list within mail and that no additional tool can do the job ! Where are located the data files of Pantheon-mail ?
You can find pantheon-mail's data in
~/.local/share/pantheon-mail/. Inside this folder you'll find a folder for each email account. Inside each account folder you'l find an attachements folder, a geary.ini file, and a geary.db file. The geary.db file can be opened with any SQLite client. DB Browser for SQLite works great:
sudo apt install sqlitebrowser. The ContactTable contains everything related to the Contacts from the elementary OS email client.
EDIT: Sequeler also works fine, and it's designed for elementary OS :)