Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
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
Eudora Internet Mail Server - Eudora Internet Mail Server is a highly advanced internet mail system that supports the all kind of email systems and clients either it is IMAP, POP3, SMTP and much more.
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.