I've been using mandrillapp.com for over 2 years now, completely free. Other people like mailgun.com. Anytime I need to setup a nas or monitoring system that needs to send out notifications, I create a new api key then add that into whatever device needs to send email.
I also for a brief period used the service as a "smarthost" for a shared mail server (not mine directly). This technically worked fine, but became an issue as end-users would get viruses and start sending spam. However, mandrill would notify you of this issue and you'd see high rejection rates. I could even view the rejected message (privacy concerns aside) to see headers of who was sending them. So while I wouldn't recommend it for a shared server, it would be fine for a personal server.