File this one in your irony folder. A Silicon Valley startup called Teneros closed a $40 million series D financing today, led by Advanced Equities. That brings the total raised since the company was founded to $84.5 million (other investors include Goldman Sachs, New Enterprise Associates, and Sevin Rosen Funds).

What does Teneros do? It keeps crashing Microsoft Exchange e-mail servers from taking down a company’s access to e-mail by providing remotely-managed “continuity appliances” that basically back up the servers. (Maybe it should consider changing its name to Tenemos, as in , “We’ve got your mail.”)

About two thirds of all companies use Microsoft Exchange for their e-mail, and even a few hours of downtime a year can cost businesses a lot of money. What I find ironic about this funding, and indeed this company, is that it is built entirely on the flaws of Microsoft Exchange. And its investors are willing to bet $84.5 million that Microsoft won’t be able to fix Exchange to obviate the need for Teneros’ product. No matter how much Microsoft itself invests in Exchange, there will always be times when it will crash. By the way, this is the same company that is going to save Yahoo.

(See also Azaleos, which raised $10 million last November).