Ok, here we go, this is what you have to do if your relay host supports TLS:
Edit /usr/share/ebox/stubs/mail/main.cf.mas. Look for this line:
relayhost = <% $relay %>
Right after this line you must add:
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/smtp_pass
smtp_sasl_security_options = noanonymous
Now you have to create a file called:
/etc/postfix/smtp_pass
And its content must be:
yourelayhost user:password
Now you have to run the follwing command:
sudo postmap /etc/postfix/smtp_pass
Final step, restarting postfix:
sudo /etc/init.d/ebox postfix restart