grommunio is described as '(formerly known as grammm) efficiently combines all requirements of modern, digital communication and collaboration' and is a Calendar in the Office & Productivity category. There are more than 10 alternatives to grommunio for a variety of platforms, including Online / Web-based, Linux, Self-Hosted solutions, Windows and SaaS. The best alternative is Microsoft 365. It's not free, so if you're looking for a free alternative, you could try Zimbra Collaboration Suite or Nextcloud Hub. Other great apps like grommunio are Kolab Community (Freemium, Open Source), Kopano (Freemium, Open Source), Kolab Now (Paid, Open Source) and OpenChange (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.
Nextcloud is the most deployed on-premises file share and collaboration platform. Access & collaborate across your devices. As fully on-premises solution, Nextcloud Hub provides the benefits of online collaboration without the compliance and security risks.
Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. It is part of the Microsoft Servers line of server products and is widely used by enterprises using Microsoft infrastructure solutions.
IceWarp integrates all communication & collaboration features in one unified solution. Available in cloud or for on-premises deployment, IceWarp is a great tool to share and exchange all information and ideas online, offline or via mobile devices.
Kerio Connect makes email, calendars, contacts and task management easy and affordable. With Kerio Connect, you have immediate, secure access to your communications anytime, anywhere, on any device - without complexity or expensive overhead.