At EpicWin App, we use Constant Contact which is the email marketing software I would highly recommend. For a reasonable pricing plan, you'll get robust functionalities. Constant Contact guarantees delivery all the time as it checks the content of your email to prevent directing to spam folders.
Moreover, its tracking and reporting functionalities stood out for me; I can keep tabs on my drip campaigns' success or potential improvements.
I additionally like how I can integrate Constant Contact to WordPress form plugins. This functionality collects visitors' data and converts as your email recipients, which, for me, is the definition of efficient.