RFC2449 defines the IN-USE extended POP3 response code as 8.1.2. The IN-USE response code
This occurs on an -ERR response to an AUTH, APOP, or PASS command. It indicates the authentication was successful, but the user's maildrop is currently in use (probably by another POP3 client). http://www.faqs.org/rfcs/rfc2449.html In contrast to this, the POP3 login code in client_authenticate.c will send IN-USE whenever authentication was not successful because of some kind of internal failure[*]. [*] Noticed during experiments with a shell SQL database driver which only ever returns errors as reaction to any query. I happen to know of at least one (partial) client implementation (I happen to have written which is used by an iPhone application for POP3 user credential verification) which actually interprets IN-USE as it is defined by the RFC.