I'm surprised that no one here has yet mentioned "followed" or "starred" repository messaging built into systems like GitHub or BitBucket.
Our technical stakeholders (project leads, development and support managers) all follow our issue and commit update histories on their relevant projects. We have a small team (15 FTE + contractors), but this seems to work for us to
No one is measured on any of these things, but in addition to weekly status reports from the PM's, this gives a daily view into the project to at least keep everyone aware of what areas are being worked on so no one goes without visibility.
It's also helped to increase transparency across developers and contractors and our business liaisons which helps everyone be accountable to their deliverables schedules.
When combined with RSS feeds associated with specific repositories or across our entire organization, we've been able to limit emails (where wanted) and offer a similar set of data in real-time and in summary via RSS readers. For some users this is Outlook so it's basically email for them, though slightly different, but for other users they use a full-fledged RSS client with all the extra filtering they need to customize it to their exact needs.
We ran into similar concerns about email volume at first, but our end-users came up with the RSS system without the Engineering Org having to do much besides suggest clients for those not using Outlook. Worked for us, again around 20-30 FTE + Contractors throughout the year across multiple offices and time zones. YMMV, obviously.