You can use a TLS connection to the local email server. You can even enforce that a mail server you run uses TLS to the destination MX. There's no way of forcing that an email will use a secure connection for it's entire journey.
Sure there is; PGP or S/MIME. Use end to end encryption and then you won't have to worry about what happens in the middle.
Can you refuse sending email to servers not supporting TLS?
I don't know the penetration of TLS in SMTP servers, tried some google-fu but I didn't find anything interesting.
EDIT: chances are I misread your comment but still I really wonder how popular TLS is.