Sparkpost - Deliver your app and website emails on time and to the inbox. Trusted to send 25% of the world's non-spam email. Email delivery for companies big and small.

SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.

SocketLabs - Marketing & transactional email delivery platform

EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.

Return Path - Inbox placement solutions from Return Path helps improve email deliverability and increases email marketing ROI.

RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.