The core bits are open source, see Mer Project for details. One fully open project building on the same core is Nemo Mobile.
Only the UI, Android Integration and first party apps are proprietary, though the new Jolla owners are working on open sourcing their apps at least.
(There was some legal drama with their Russian investors, the original Finnish founders managed to get the rights to everything back after the investors stopped responding)











It’s unfortunately how gmail handles IMAP. I don’t think any clients handle that differently