I use Exim and Heirloom-mailx (although you can use a different user program, since the server is only Exim). On the Debian setup menu, I selected smarthost, t

I use Exim and Heirloom-mailx (although you can use a different user program, since the server is only Exim). On the Debian setup menu, I selected smarthost, to use the ISP's server for sending (required because of the way the internet service works; your own service is still used for receiving). And then, in order to reduce spam, modified the configuration so that only aliases can be used and not real usernames, and set up several aliases in the /etc/aliases file, so that a different one can be used for each service or correspondent. I then set up the router to allow incoming SMTP connections. (If necessary, you may need to disable NAT with your internet service provider. If they won't let you to do this, or won't allow arbitrary port numbers, then it isn't a real internet service.)