day3 API
day3 is an email platform billed by the emails you send, never by the size of your list. This API is how your code uses it.
There are two things it does:
- Send transactional email. Password resets, receipts, magic links. One
POST, out of the same verified domain and the same monthly allowance as your newsletter. - Manage the list. Audiences, contacts, custom fields, segments, topics and suppressions, built so that moving a list off another provider is a short script rather than a project.
Campaigns can also be drafted, previewed, tested and sent over the API, automations can be enrolled into from your own backend, and the same surface is exposed to AI editors through an MCP server.
Base URL
https://go.day3.app/api/v1The app is served from go.day3.app, so the API is too. day3.app is the
marketing site and has no API.
Your first request
curl https://go.day3.app/api/v1/audiences \
-H "Authorization: Bearer $DAY3_API_KEY"If that returns a JSON object with a data array, you are done setting up. The
quickstart goes from nothing to a delivered email in three steps.
How to read these docs
| If you want to | Start at |
|---|---|
| Send your app’s operational email | Emails |
| Move a list off Resend, Mailchimp or Kit | Migrate a list |
| Keep your users in sync with an audience | Contacts |
| Know when mail bounces, without polling | Webhooks |
| Write and send a newsletter from code | Campaigns |
| Start a welcome series when someone signs up | Automations |
| Let Claude Code or Cursor draft an email | MCP server |
Read Conventions once. Pagination, idempotency, ids and rate limits work the same way on every endpoint, and they are not repeated on each page.
Objects
| Object | Id prefix | What it is |
|---|---|---|
eml_ | One transactional email sent through the API. | |
| Audience | aud_ | A list of contacts. Everything else lives inside one. |
| Contact | sub_ | A subscriber. Unique by email within an audience. |
| Field | fld_ | A registered custom attribute key. |
| Segment | seg_ | A saved filter, evaluated live at read time. |
| Topic | top_ | A subscription category a contact can opt out of on its own. |
| Campaign | cmp_ | A newsletter: one email to a whole audience. |
| Automation | aut_ | A published flow a contact runs through, one step at a time. |
| Suppression | sup_ | An account-wide “never email this address” entry. |
Set up in the app
Sending domains, senders and OAuth clients are configured in the app rather than over the API, and automation flows are built on the canvas in the app and only enrolled into from code. Verify a sending domain under Sending before you mail real subscribers.