The problem is, Inbox already has a BUNCH of VC's behind it. Which means a lot of people only worried about making money on the board. Unless the founders collectively agree they won't sell the company and also have a majority of shares in the company, the first big tech company that comes knocking with an offer will treated like the second coming.
Tens of millions and being in charge of a company that improves lives for millions of people around the world in a way you'd love to keep heading up.