Inbox Ignite - Prevent your emails from going into spam
MailerSend - Better transactional emails for teams.
MailClark - The Slack bot for external communications
PigeonBot - Send emails via Slack with the /email command
Really Good Emails - A large collection of good product email design.
Mailshake - Mailshake lets you build your professional network through a smart and intuitive approach to cold emailing.