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.
Wine - Open Source Software for running Windows applications on other operating systems.
Mailivery - Reach the Inbox, not the spam folder.
CodeWeavers CrossOver - Don't buy a Windows license, don't reboot and don't use a Virtual Machine. Try a free trial of CrossOver to run your Windows software on Mac, Linux and Chrome OS.
Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.