Intuit, the company behind the well-known Quicken suite of money management software that includes Quicken Online, can’t believe how well its competitor Mint is doing. In fact, they were so bewildered by Mint’s claims of gaining 3,000 new users a day and jumping from 600,000 to 850,000 users in a matter of months that they decided to send a threatening letter demanding an explanation for this apparently inconceivable feat.

We’ve obtained copies of both Intuit’s letter and Mint’s reply, which we’ve embedded below. From Intuit’s letter:

Note that the letter says that Intuit doesn’t "wish to suggest that Mint is engaging in false advertising", despite the fact that that was the entire purpose of the letter. Nice.

Aside from Mint’s remarkable growth rate, Intuit’s concerns focus on the startup’s definition of "users". Mint’s reply states that the company considers anyone who has filled in an Email address, Zip code, and password to be a "user", regardless of whether they’ve ever actually linked their bank account to their Mint user name. This figure obviously overstates the number of people who actually use Mint on a day-to-day basis, but it’s also a fairly standard way to define "users" for most web services. As an outside check, Compete counts 416,000 monthly unique visitors worldwide, and growing rapidly (see chart below).