Some of it is mail in ballots but that's normal in most states. The 1 week delay in Mississippi right now are the affidavit (provisional) ballots where someone didn't have the right ID, had just changed their name, or otherwise couldn't match their registered voter identity.
Those folks sign a temporary vote (affidavit or provisional) and then have a week to follow up with the proper ID and otherwise sort out their information to ensure their vote is counted.
