Install claws-mail-address-keeper by entering the following commands in the terminal:
sudo apt update sudo apt install claws-mail-address-keeper
Address keeper plugin for Claws Mail
This plugin allows saving outgoing addresses to a designated folder in the address book. Addresses are saved only if not found in the address book to avoid unwanted duplicates. . Selecting which headers are scanned for keeping addresses is also supported, one or several of 'To', 'Cc' and/or 'Bcc'.