Mobile mails optimisation
When a user send a mail from his/her mobile device, the mail is transfered twice over the network: once to send it to the recipients, and once to save it in the user's sent items on the server. The goal was to have is transfered only once over the network and still have it save in the user's sent items on the server.
This solution was implemented with Exim. Though this might seem trivial to implement, there was a catch with BCC headers which needed a configuration tweak.