There are free mail servers available for any OS you can think of, and the modules specified above will interface well with them. The main problem is not sending emails, but rather preventing them from being filtered out at their destination. A lot of hosts reject emails that come from a dynamically assigned domain name / IP. If you're going to be doing a lot of mailings (and I hope they're legit, not spam), you'll need a static IP with a domain name pointing to it, that you can reference in your From / Return-Path header fields and that the target host can verify using reverse lookup.