​404 Resource Not Found:

Mailbox Not Found

• If you are encountering this error message during credentials validation, the email address does not correspond to the primary SMTP address of a mailbox.
  

Folder Not Found

• If you are encountering this error message in the middle of a migration, the folder we pr​eviously discovered was renamed or deleted.
  

Item Not Found

• If you are encountering this error message in the middle of a migration, the item we previously discovered was renamed or deleted.