[exim] How can I require TLS for mail submission?

It works fine now for voluntary TLS mail submission. What do I have to do
to make it mandatory? With all of the mobile devices these days, this
should be the most asked question on the list. Users are giving away their
user name and password all day long at hot spots in coffee shops and
airports if they are not forced to use TLS like all of the large E-mail
services require these days.