Modified by message center means it is transactional delivery. Last
Modified: as a lot of Campaign objects, is the date when the elements
have been modified (i.e., the line on the table has been modified, with
at least 1 column change). This date changed several times after the
last message: the recipient's activity lasts weeks (months ?), and new
trackings appear (open, click, etc.), so the delivery indicators are
updated, implying a lastModified update.
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