Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Xero - Xero is online accounting for your small business.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Datamolino - Process all invoices without retyping. We turn your invoices into structured electronic documents, that you can import directly into your accounting system.
mailcow - An open source mailserver suite.
DOKKA.com - The Future has Arrived: Accounting Process Automation