PRE-$1M ARR — SELL THE MEASUREMENT, NOT THE SENDING

In marketing infrastructure, the payable job is proving what marketing did, not doing what marketing does. Lead scoring, attribution and pipeline contribution are what buy budget; email sending is a commodity that funds nothing.
Enter through the buyer whose budget is under threat. Marketing leaders under pressure to justify spend will fund the tool that produces the defence.
Build the CRM integration as the product's core rather than as a connector. Your value is created at the handoff between marketing and sales, and the handoff is the moat.
REFUSE: SMB pricing. B2B marketing automation was built on enterprise contract values and long cycles; the SMB version of this category is a different company.

$1–5M ARR — WIN THE COMPLEX SALE, NOT THE SIMPLE ONE

Serve organisations with long, multi-touch, multi-stakeholder sales processes. Your product's complexity is only justified where the customer's process is complex, and it is unsellable where it is not.
Charge on contacts in the database with a committed annual floor, so the price grows with the customer's list automatically.
Build the services and implementation practice knowingly, then push it to partners as fast as you credibly can. Marketing automation carries structural implementation drag.
WATCH: time from contract to first campaign sent. It is the leading indicator of both churn and reference-ability.

$5–10M ARR — BUILD THE CERTIFIED PRACTITIONER ECOSYSTEM

Certify consultants and agencies so an ecosystem of people whose careers depend on your product grows outside your payroll. In complex enterprise tools, the practitioner community is the switching cost.
Make the product a line on a marketer's CV. Skills-based lock-in outlives feature parity.
Publish benchmark data on the category so the market's vocabulary is yours.
WATCH: certified practitioners in the market, and job postings naming your product.

$10–50M ARR — DECIDE WHETHER YOU ARE A COMPANY OR A MODULE

Recognise the strategic truth of this category early: marketing automation is a feature of the enterprise stack, and every CRM and ERP vendor will eventually need one. That makes you a highly desirable acquisition and a difficult standalone.
Build the piece the platform is structurally awkward about building — deep integration across competing CRMs — because that is precisely what makes buying you cheaper than building.
Go public if the window is open and you need the credibility, but treat it as a step, not a destination. (Eloqua listed on NASDAQ in August 2012.)

$50–100M ARR — SELL INTO A CONSOLIDATION WAVE, NOT AFTER IT

Time the exit to the moment the platform vendors are competing to own your category, not once one of them has already bought a competitor. (Oracle agreed to acquire Eloqua in December 2012 for approximately $871M — roughly four months after the IPO, and amid an active consolidation wave in marketing software.)
Understand what the premium is paying for: an installed base of enterprise marketing departments and a certified practitioner ecosystem, not the software.
Negotiate for the integration roadmap. The value you built is destroyed if the product becomes a checkbox in a suite.

$100M+ ARR — INSIDE A SUITE, THE RISK IS NEGLECT, NOT COMPETITION

State the outcome honestly: Eloqua's independent life ended at roughly $100M of revenue. Inside Oracle it became a component of a marketing suite, and the category leadership it held in 2012 did not survive as a standalone brand advantage.
The transferable instruction for founders: if your product is a likely module in someone else's platform, the highest-value decision you make is the timing of the sale, not the product roadmap.
For anyone acquiring or being acquired, assume that suite-owned products lose their category identity within three to five years, and price the deal accordingly.