Sidemail - The simplest way to send transactional emails. Sidemail makes sending transactional emails trivial, it's perfect for side projects, MVPs or the next big thing.
Mailook - Identify & fix your email deliverability issues for free 👀
Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid
Mailivery - Reach the Inbox, not the spam folder.
YaySMTP - WordPress Mail SMTP
The Sales Email Deliverability Guide - An in-depth guide to understanding email deliverability and how to improve it.