Rated 5 out of 5 stars

Edit: I found what was causing the problem: the mail header field *must be in lowercase* in the extension options, even if it is in mixed case in the message header.

Fields created with the extension are not populated when importing mbox files using the ImportExportTools addon.
Column is active in the folder that will receive the messages, with the "Apply columns to / Folder and its children" option enabled.

This review is for a previous version of the add-on (6.1.2).