HTML-only email measurably scores worse for spam filters

Sending only an HTML part and skipping multipart/alternative's plain-text part is a small, easy-to-miss omission that measurably hurts spam scoring.

This is one of the things Transactional Email Deliverability Core already handles. Send transactional email that actually lands, and stop emailing people who bounce or complain.

Buy for $793 of 3 clean-room builds passed · full refund if it fails on your machine

Is this you?

An email sent with only an HTML body, no plain-text alternative, is missing half of what spam filters expect from legitimate transactional mail -- multipart/alternative with a real text part is a signal filters weight, and its absence measurably contributes to worse spam scoring. The email may still arrive in the inbox most of the time, which is exactly why the gap tends to go unnoticed until deliverability degrades for reasons that look unrelated.

Why this one is easy to get wrong

Building an HTML email template is the visible, testable part of the work -- it renders, it looks right in a preview -- and a plain-text part produces no visual output to check, so it's the kind of thing that's easy to skip entirely without any test or manual check ever catching the omission, since the HTML-only email still "works" by every obvious definition.

What you get instead

The pack's template registry contract requires every template to return a non-empty text field alongside subject and html -- it's part of renderTemplate()'s return type, not an optional extra, so a template that only fills in HTML fails the type contract before it ever reaches a send. Both shipped templates (welcome, password-reset) include a real plain-text part written for the email, not auto-stripped HTML tags.

Source: ARCHITECTURE.md 'Templates' — checkable in the pack you receive

How you actually use this

You don’t install a library or wire up an SDK. Your own coding agent builds the code in your project, and you keep it — no runtime dependency on us.

Step 1
Download and unzip
You get a folder: the docs that tell an agent what to build, a starting skeleton, and the test suite that decides when it's done.
Step 2
Open it in Claude Code or Cursor
Point your coding agent at the folder. Nothing to install, no account with us, no API key.
Step 3
Paste one prompt
The pack contains the exact prompt. Paste it as your first message and leave it alone — it works through the build itself, choosing a cheaper or stronger model per task.
Step 4
Run ./verify.sh
One command. It prints a pass or fail for every check. Green means the build is done — the same script we ran to produce the receipt on this page.

Typical build: about 13 minutes of your agent working, mostly unattended. Then you integrate the working module into your app the way you would any code you’d written yourself.

Why you can believe this

3 of 3 runs passed

We ran this pack from an empty folder 3 times and published exactly what happened — every check, the model, the token cost, the wall time. Not a testimonial, and not our opinion: the same verify.sh you run yourself. Read the full receipt →

Buy for $79 14-day refund if verify.sh fails →