It'd be great if we could expand on these a bit since this isn't all obvious and I had to look this up to review it.  So I propose using the following comment as a starting point, or just using it:

```
/*
 * Transient failures:
 *
 * LOGIN-DELAY: RFC2449 defines the LOGIN-DELAY capability to tell the client
 * how often it should check and introduces it as a response code if the client
 * checks too frequently.
 * See http://tools.ietf.org/html/rfc2449#section-8.1.1
 *
 * SYS: RFC3206 provides disambiguation between system failures and auth
 * failures.  SYS/TEMP is something that should go away on its own.  SYS/PERM is
 * for errors that probably require a human involved.  We aren't planning on
 * actually telling the user what the SYS/PERM problem was so they can contact
 * tech support, so we lump them in the same bucket.
 * See http://tools.ietf.org/html/rfc3206#section-4
 *
 * IN-USE: Someone else is already in the maildrop, probably another POP3
 * client.  If optimizing for multiple POP3-using devices was something we wanted
 * to optimize for we would indicate a desire to retry here with a more extensive
 * back-off strategy.
 * See http://tools.ietf.org/html/rfc2449#section-8.1.2
 */
```