The Re-Mail option is useful for dealing with returned letters from bulk mailings or creating ad hoc labels. You can create new mailing labels for only those contacts once you correct their address. This will allow you to send them the mailing again.
You can mark a contact for re-mailing by going to their Contact screen record and checking the Re-mail check box. This box is located on the Contacts tab. Use this check box to mark a contact whose mail was returned and whose address has been corrected.
Re-Mail is also useful for ad hoc labels and merge data. When you only want labels or to export data for a few contacts, you can check the Re-Mail box for only those contacts and then produce labels or export data for them.