Authentication is not required to exploit this vulnerability.
The specific flaw exists within the POP3 and IMAP services while processing malformed e-mails. The vulnerable code expands specific non-printable characters within a "mail from" command without allocating adequate space. By providing enough of these characters, memory can be corrupted leading to arbitrary code execution under the context of the SYSTEM user.
Disclosure Timeline:
2008-08-26 - Vulnerability reported to vendor
2011-02-07 - Coordinated public release of advisory