What's the difference between M_EMAIL and M_NEWEMAIL? MY hunch is this is used when users want to change their email address? You save M_EMAIL as M_NEWEMAIL until they confirm the change?
M_EMAIL holds the user's current e-mail address. M_NEWEMAIL holds the user's new e-mail address when they change it. After the person clicks on the the link in the e-mail they receive, to confirm that the e-mail address they entered is valid, the value in M_NEWEMAIL is moved to M_EMAIL.
M_DATE is the date the Member registered on the forum, it never changes.