This tool is made by the french OSS company Linagora, active in that space for more than 20 years.
They mostly likely have thousands of users running on corporate or state platforms so it's not a "one off project" supported by a couple individuals, it's an actual stack with probably a large dev team.
However, Linagora has been entangled in a legal battle for 10 years with former employees that founded BlueMind[0], a competing offer. Latest episode in the saga appears to be "back to square one" with the legal case coming back to a fresh start based on complex legal issues [1]
[1]https://www.zdnet.fr/blogs/l-esprit-libre/blue-mind-linagora...