The Email They Almost Deleted: The 3 Metrics That Made Mailchimp a Giant (And That Your SaaS Is Probably Ignoring)

3 metrics separate a SaaS worth a fortune from one that barely survives. It’s not user count, not traffic, not how many LinkedIn posts you publish about your launch. It’s three numbers that most bootstrapped founders—in Spain and everywhere else—don’t even calculate.

Before diving in, let me tell you something.

The story of the email they almost deleted

In 2001, Ben Chestnut and Dan Kurzius ran a design agency that was doing fine. They launched Mailchimp almost as a joke: a side tool so clients could send newsletters without bothering them.

It could have died right there.

For years it was secondary. No external investment. No funding rounds. No typical startup noise. Just two guys building something useful and charging for it.

Twenty years later, Intuit acquired them in one of the largest business software deals in history, and the founders kept 100% until the day they sold.

The question nobody asks: what were they measuring during those 20 years that made the company worth so much?

Spoiler: it wasn’t Twitter followers.

Metric 1: NRR — The number that decides everything

Net Revenue Retention (NRR) measures how much money your existing customers retain year over year, including upgrades, downgrades, and cancellations.

The difference between bands is brutal:

NRR above 120% → valuations of 8x revenue
NRR below 90% → valuations of barely 1.2x revenue

That’s a 6.6x difference in what your company is worth based on a single number.

The global SaaS market median sits at 102% NRR. The strongest public companies average around 114%.

What NRR above 100% means: your existing customers pay you more this year than last, without acquiring a single new one. Your base grows on its own. That’s what investors call expansion revenue, and it’s why some bootstrapped SaaS companies like Mailchimp can ignore the external funding noise for decades.

How to calculate it today:

If your NRR is below 100%, every month that passes, your customers collectively pay you less. That’s a slow hemorrhage many SaaS founders don’t detect until it’s too late.

100-110%: good, but there’s room for improvement.

Above 120%: you’re in the territory where valuations skyrocket.

Metric 2: Rule of 40 — The health test nobody runs

The Rule of 40 combines two things into one number: growth rate + profit margin. If the sum exceeds 40, your SaaS is healthy.

The market reality in 2026:

Median SaaS company: 12% (10% growth + 6% EBITDA)
Doximity, one of the leaders: 55%

Twelve percent. That’s what the typical company achieves. Not the one in crisis: the median.

This means if you’re building a bootstrapped SaaS without aggressive scaling for a year, you’re probably not as far below market as you think. The real benchmark is much lower than Twitter gurus make it seem.

But it also means there’s a massive gap between 12% and the 55% of top performers. That gap is where companies that actually matter get built.

How to calculate it:

If your SaaS grew 35% this year with a 10% margin, your Rule of 40 is 45. You’re above the threshold.

If you grew 15% with a -5% margin, you have a 10. That signals you’re burning cash without growing enough to justify it.

Metric 3: LTV:CAC — Where most founders invest backwards

LTV:CAC measures how much a customer is worth over time compared to what it cost to acquire them. The sustainable benchmark is 3:1.

The problem in 2026: acquiring a new ARR customer costs, on average, double what it generates. That ratio has worsened 14% year over year.

And here’s the most interesting part:

Expansion revenue costs approximately half as much as acquiring a new customer.

That’s a massive inefficiency that most bootstrapped SaaS founders ignore because they’re obsessed with acquisition: more ads, more SEO, more cold outreach.

Mailchimp understood this before anyone else. If you build a product that users use more as their business grows, revenue climbs on its own. You don’t need to convince anyone new: the people you already have become your growth engine.

In practice, this means:

Plans with limits that scale with usage (emails sent, contacts, seats)
Premium features users want when they grow, not ones you force them to contract
Onboarding that maximizes early activation (if they don’t use the product in the first 7 days, they won’t pay in month 2)

The SaaS market in 2026: why these numbers matter now

The global SaaS market has been growing at 18.7% annually and is projected to reach astronomical figures in the coming years. That attracts competition, but it also means multiples for companies with solid metrics remain elevated.

For those of us building SaaS from Spain or Latin America without external investment, this matters for two reasons:

You don’t need hypergrowth to create real value. Mailchimp took 20 years. Basecamp has operated for decades with around 50 employees generating revenue that most small businesses would envy. The “you must grow fast” benchmark is set by the VC ecosystem, not market reality.
If you ever want to sell, metrics are the price. High NRR and a healthy Rule of 40 give you negotiating power. Low NRR turns you into a business worth only your current MRR multiplied by a modest number.

What you can do this week

Straight to the point—here’s what I’d do in your position:

1. Calculate your real NRR. Not the one you think you have: the one you actually have. Open your Stripe or Baremetrics dashboard and see what percentage of MRR from 12 months ago is still active today, plus expansion.

2. Calculate your Rule of 40. Add your annual revenue growth rate and your operating margin. Below 20, you have an efficiency problem. Below 10, you have a crisis you haven’t named yet.

3. Identify your main expansion revenue lever. What feature or limit makes your best customers pay more over time? If you don’t have an answer, that’s your next product problem.

Mailchimp wasn’t an overnight success. It was 20 years of measuring the right things, retaining customers better than anyone else, and not getting distracted by startup ecosystem noise.

The email they almost deleted ended up being the most valuable business they ever built.

The metrics were there all along. The question is whether you’re measuring them.

We keep building.