When you take 4.35 million in investment, you are giving away a percentage of your company. Say it's 1/3rd for the sake of argument. That creates a post-money valuation of $13m. Which means that a $20m buyout (once the lawyers get their slice) is, what-- a 1.5x return on investment? The only way an investor would approve that is if the company was tanking (Xobni isn't-- they are buried in good press).
Outlook (and email) is the #1 technology time-sink on the planet (RescueTime has the data to prove it-- someday we'll publish some interesting stats).
If you could walk up to any enterprise and propose to make email 10% more efficient, that's worth a TON of money.