We all know that the best way to increase the deliverability of your emails sent from your website is to use a third-party SMTP gateway that can add extra layers of authentication (proving you’re not spam) and protection against being blacklisted. (Read Email on Pantheon for more info.)
In that doc we mention SendGrid for these transactional emails. I know some folks who use Mailchimp (Mandrill), Amazon SES, or the same SMTP gateway that you already use for sending and receiving emails. Personally, I’ve used MailGun for years and been pretty happy.
What service do you use and why? Do you use that for every project or just under certain conditions?