mailcow - An open source mailserver suite.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Mailgun - A set of powerful APIs that enable you to send, receive and track email from your app effortlessly whether you use Python, Ruby, PHP, C#, Node.js or Java.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
Brevo - Innovative online Email Marketing solution to manage your contacts, create & send your newsletters and track your results. More than 80 000 clients. Best prices and attractive features.