SaneBox services require your email client software to be set to view your email with IMAP or OWA or WebDav or ActiveSync.
POP is a very old protocol that simply downloads your latest email to your local computer, ignoring the server for any other purpose.
Unfortunately, because your SaneBox folders like @SaneLater are held on your mail server, you can't see them when using POP server settings.
Most email servers and email clients support IMAP. For those Exchange servers that don't support IMAP - they usually support either OWA or WebDav or can work with SaneBox via ActiveSync.
Here are rare examples of providers that only offered email by POP for a very long while after the rest of the world had IMAP provided for their email accounts. Thus, most users of these services will still be set up in their email software with the limits of old-style POP.