Enabling STARTTLS

As a Server
Exim Configuation
     tls_advertise_hosts = *
     tls_certificate = /usr/local/etc/exim/certs/hostname.crt
     tls_privatekey = /usr/local/etc/exim/certs/hostname.key

As a Client
Happens automatically when server offers STARTTLS
Exim Configuation 
     begin transports
     ...
     remote_smtp:
       driver = smtp
       tls_certificate = /usr/local/etc/exim/certs/hostname.crt
       tls_privatekey = /usr/local/etc/exim/certs/hostname.key