The best free alternative to iRedMail is mailcow: dockerized, which is also Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to iRedMail and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to iRedMail are Mail-in-a-box (Free, Open Source), mail.com (Free), Zimbra Collaboration Suite (Freemium, Open Source) and Horde (Free, Open Source).
Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO compared to legacy platform vendors.
Horde Groupware is a free, enterprise ready, browser based collaboration suite. Users can manage and share calendars, contacts, tasks and notes with the standards compliant components from the Horde Project.
Cozi is a free online calendar that helps families coordinate activities. See who needs to be where and when, be reminded of upcoming appointments, and print your calendar by week or month. Cozi is the easiest online calendar you'll ever use.