Microsoft's Exchange Team published a blog post on August 8, 2024, about decommissioning Exchange Server 2016. Although the intention was definitely not to make people switch to an open source groupware, the announcement was enlightening:

Exchange 2016 is approaching the end of extended support and will be out of support on October 14th, 2025. If you are using Exchange Server 2019, you will be able to in-place upgrade to the next version, Exchange Server Subscription Edition (SE), so Exchange Server 2016 will need to be decommissioned at some point. If you plan to stay on-premises, we recommend moving to Exchange 2019 as soon as possible. [1]

A long list of todos and caveats follows, including reboots after upgrade. No wonder Microsoft's general strategy toward its cloud (Microsoft 365 and Azure) is worrying users, including many groupware administrators who are currently looking for other options. Not so long ago, Australian vendor Atlassian took a similar approach, attempting to force their customers – users of Confluence, Trello, and Jira – into the vendor-owned cloud. Like Atlassian customers, now more and more Exchange administrators are checking the market, and vendors of open source alternatives are reporting growing numbers of customers.

However, what's equally stirring up the groupware market is security and data protection. Over the past few years, several severe issues popped up in or around Microsoft Exchange or its Azure Cloud. After Chinese hackers stole a master key to Microsoft's Azure cloud, the US government did not spare harsh words, calling out Microsoft for "shoddy security, insincerity" and "cascades of security failures" [2].

MAPI: The Curse of Exchange Protocols

Under the pressure of new European Union regulations such as the General Data Protection Regulation (GDPR) or the Cyber Resilience Act (CRA), it's no wonder customers are browsing alternatives. As long as you're only dealing with Linux and open source clients, you will be perfectly happy to use standard open protocols – from IMAP to POP (mail) and from CalDAV (calendars) to CardDAV (contacts). Your client of choice will be Mozilla Thunderbird or KDE's Kontact, whose protocols will work fine with mobile devices, too.

However, if your users need or want Windows clients with Microsoft Outlook or a bit more sophisticated support for smartphones, then the situation is different. Outlook and Exchange communicate over a large set of APIs and protocols usually known as the Exchange protocol stack, and in most companies, Outlook is deeply integrated into a number of special applications needed for daily work. Only a few other groupware products can achieve similar features, open source or otherwise.

Vendors usually try to tackle that issue with Outlook plugins, changes to the Windows registry file, or both, but adminstrators tend to dislike any changes to the client's configuration, and both plugins and registry changes have caused problems in the past.

grommunio: Native Support for Outlook

Grommunio [3] is a startup from Austria that promises a "drop-in replacement" for Microsoft Exchange. No Outlook plugin, no registry entry, no changes to clients, and no need to sit at and click on a client's desktop during migration – so the Vienna company claims. The goal is for every Windows system and almost every smartphone (including Apple clients) to connect seamlessly through standard tools available on every modern system and talk exactly the way they would to Exchange servers. Grommunio's customers confirm that grommunio performs as advertised; Deutsche Telekom is even selling grommunio as part of its Open Source Collaboration Open Telekom Cloud [4].

A Full Stack Re-Engineered

The grommunio developers chose a completely different approach: Instead of plugins and workarounds, they have been (re-)implementing more than 50 protocols and APIs of the Exchange stack in open source and have published all of their work on GitHub. Thus, a grommunio server speaks exactly the Exchange language its Outlook clients expect – natively, but in open source. (See the "From the FSFE and EU" box.)

Because grommunio behaves as an Exchange server would, you simply have no need for a risky plugin or quirky registry entries. Not only is grommunio open source, it also builds on a huge set of trustworthy, renowned, and proven equally stable and secure open source software in the back end.

In tedious work spanning more than four years, reading thousands of pages of specifications from Microsoft and testing and occasionally correcting and amending them (Microsoft was always helpful and cooperative), the grommunio developers managed to create an open source server application (gromox) within grommunio that speaks Exchange protocols natively, with a wide range of clients.

Accomplishing that feat took not only a lot of time but also patience, as grommunio's lead developer Jan Engelhardt demonstrated in his presentation at FOSDEM'24 in Brussels (Figure 1), where he went into many, sometimes funny, details of his work [6].

Figure 1: Grommunio knows a steadily growing number of Exchange protocols. This list is from a presentation at FOSDEM'24 by Jan Engelhardt [6].

Many Windows and Exchange administrators know the Messaging Application Programming Interface (MAPI), but that term is "somewhat ambiguous," explains Engelhardt. "It is used for concepts as well as for the data mode, programming interfaces, and network protocols on the wire." The grommunio website continues:

Thanks to Microsoft, all documentation is freely available, the specifications amount to "132+ documents on 8400+ pages, in addition to the Internet mail protocols (i.e. RFC 5322, 5545, etc.) that must be supported anyway."

Engelhardt and his team at grommunio dived deep into these documents and also helped to fix some problems. This is how grommunio became a contributor to the open stack of specifications used by Microsoft and all its customers. [7]

MAPI and Exchange isn't the only language grommunio speaks (Figure 2). Autodiscovery is used for client configuration (just enter your email address and DNS and the server will do the rest for you); Exchange Active Sync (AES) syncs and controls your mobile phones; and Exchange Web Services (EWS) in the world's first open source implementation that connects not only Apple clients but also Linux programs such as Evolution, Thunderbird, and KDE Kontact with the grommunio server [8]. Also, grommunio Meet (a Jitsi implementation), grommunio Chat (Mattermost), grommunio Files (Nextcloud), Archive, Antispam, and directory management (LDAP, Samba) are all integrated and based on standard tools like Postfix and NGINX.

Figure 2: Grommunio is more than just groupware and integrates many other open source tools, from Nextcloud to Jitsi and Mattermost. It also offers archiving, chat, video conferencing, and mobile device management. © Jan Engelhardt, FOSDEM'24 [6]