STARTTLS isn't just "optional encryption". For example, my email server will accept any email encrypted or not, but will only allow authentication over a chann

STARTTLS isn't just "optional encryption". For example, my email server will accept any email encrypted or not, but will only allow authentication over a channel if it's encrypted. But encryption is not optional for the latter case. Email clients that attempt to authenticate won't send passwords over unencrypted plain-text connections, so there's no real vulnerability, unless a client is broken.