My name is Kostya Vasilyev and I was the original developer of an Android email app called Aqua Mail, sold to MobiSystems in 2019 at 3.5 organically grown users.
After that, I built a new email app called Sugar Mail, based on a cloud email sync platform. The project hasn't been a success with end users, but I think the sync platform could be useful to someone so I'm looking to sell it and include the app too.
Mail sync runs in the cloud and supports IMAP and Exchange, both with push mail, and the latter with bi-directional calendar and contact sync. Both Office 365 and on-prem Exchange are supported. Running mail sync in the cloud allows the Android app to be very efficient in terms of battery use.
The Android app is just a JSON RPC client with some UI on top - so building an iOS or a desktop app would be pretty easy for the buyer.
There are several geo-distributed server nodes and the Android app uses the closest one. Adding nodes for capacity is easy.
The project is actively maintaned, with the Android app optimized for Android 17. The server is written in Go. I recently rewrote it in Rust, but decided to stay with Go in production for a variety of reasons (would be happy to explain in more detail).
Unit economics: a $10 VPS can handle 1000 users each averaging 2 email accounts.
A comparable platform is Nylas which charges $15/month per 10 accounts and has no end-user app (they used to, but not anymore).
More details here: https://sugarmail.app/for-sale-one-pager.html
Happy to answer questions.