claws-mail-address-keeper - 3.17.8-1+b1 main

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.
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Selecting which headers are scanned for keeping addresses is also
supported, one or several of 'To', 'Cc' and/or 'Bcc'.