Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.
mailcow - An open source mailserver suite.
Creativity 365 - Cross-device content creation suite
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.
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.