Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid
thankme - Create a thank-you page for anyone and express gratitude
Mailook - Identify & fix your email deliverability issues for free 👀
GiveThanksNow - Send thank you notes to COVID-19 frontline workers
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.
NumWorks - The graphing calculator that makes learning math easier.