Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
mailcow - An open source mailserver suite.
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.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
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.