The Yahoo! External POP Client uses the POP3 protocol (RFC1939), including the UIDL command.

For the best user experience, we strongly recommend the implementation of the optional UIDL command from the POP3 specification and the extended response codes in RFC 2449 and RFC 3206. That said, we still do use the LAST command from RFC 1460 for servers that do not support the UIDL command.