Mailivery - Reach the Inbox, not the spam folder.
Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid
GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!
Mailjet - All-in-one email service provider that provides maximum insight and deliverability results for both marketing and transactional emails.
Flowmailer - The email delivery platform you can rely on
PromoTab - This tiny tool uses a real Gmail account to check if your emails will land in the Primary inbox of a reader or if it's stuck in the promotions tab or SPAM folder.