POP3S vs POP3 - What's the difference?
Can you tell me if there are any differences between POP3S and POP3 with STARTTLS? What's the safest? Are there any known attacks? If you could choose between them, which would you pick?
The only difference between POP3S and POP3 is that with old-style, it is implied that you will start SSL negotiation immediately upon connection. With STARTTLS, you go through the same process, but only after issuing/receiving a STARTTLS command. The protocols are designed to throw away any information discovered or negotiated before the connection was secured, and start over again from scratch.