What does “DUP” mean in the mailing list “SendStatus” column?
“DUP” means that some address in this TO field already appeared in a previous TO field and was previously sent during this CustomMailer session and you have the “Don’t send twice to same TO address” option selected in Preferences. If so, the message is not sent to anyone for this mailing list row. Addresses appearing in this or previous CC or BCC fields do not cause duplicates to be detected.