Using an external service for delivering your emails can take a lot of load off your shoulders if deliverability is a must. Services like Sendgrid and Postmark have pre-configured, whitelisted servers so that your email ensures passage by spam sifters. However, such services are generally pretty expensive, too, especially if you are a high-volume sender.
Do you use an email deliverability service? If so, what service? Why or why not?