From time to time I send on to my wife, who sits opposite, or she sends on to me, an email I (or she) received but she (or I), for some reason, wasn’t sent. Up to now, such mails went off site and then came back again, which always struck me as unsatisfactory.
So now I’ve implemented a small POP3/SMTP server which runs on a lergely-unused file server we have here. It listens on two ports and will accept mail from users who are registered to it, store it, and serve it up when asked to. This turned out to be quite easy to implement, with two listeners which pass requests off to sockets which then spawn a thread to do the work. Works nicely, and was easier than I expected.