Is there any kind of flag set on an email msg stored on a POP3 server that
indicates whether or not the msg has been "read"?

i.e.: if that msg is accessed (but left on the server) via a client such as
webmail, zerofootprint text only email client, or even via Telnet, will I be
able to determine later that it has been "read" (opened but still on the
server)?

Is seems that anyone with the appropriate account info would be able to
access a msg. Is there a way to determine if that has been done?