I had LastSent (or NumberSent) turned on, then I turned it off, but it is still there, why?
When you turn LastSent and/or NumberSent on in Preferences, corresponding columns are added to your mailing list if they are not already present, and after that the values in these columns will be updated accordingly for any mailing list rows that are sent. If you then turn off LastSent and/or NumberSent, that means that CustomMailer simply stops updating these columns, but it does not remove them. You can later turn them back on and resume updating them for subsequent sending of mail. If you actually want to remove these columns after turning them off in Preferences, use Delete Mailing List Columns under the Edit menu.