From Resend

Start an import from the dashboard's Import page (see the overview) with a Resend API key that has full access. A sending-only key can't read your account, and the import refuses it.

Resend shares one rate limit between all of a team's keys, your live sending included, so imports read your account at a fraction of it. Contacts are the slow part. Resend returns a contact's custom properties and topic choices one contact at a time, so with properties or topics an import brings over a few thousand contacts an hour.

What's imported

Not imported: broadcasts, API keys, a template's from and reply-to addresses (pass them with each send; the report notes which templates had them), domains' open and click tracking settings (see tracking), and webhook signing secrets.

Suppressions

Resend's suppression API isn't available to every account. Where it is, bounces, complaints and manual entries become suppressions for every email (scope *) with the same reason. Where it isn't, the import says so: export the list from Resend's dashboard and import the CSV on the Suppressions page (importing). Contacts who unsubscribed are imported as unsubscribed contacts, which marketing email skips.

Topics

A topic's key, which is what a send's category names, is made from its name in lower case: Product updates becomes product_updates. Its default subscription and visibility are kept, and so is each contact's opt-in or opt-out.

Templates

Resend templates fill {{{VARIABLE}}} placeholders. The import converts them to coss.com's template language and declares every variable the Resend template had:

• {{{NAME}}} stays raw in the HTML and becomes {{NAME}} in the subject and text, which are never HTML-escaped anyway.
• A variable's type (string or number) is kept. Its fallback becomes its default; one without a fallback is required, as it was in Resend. {{{NAME|fallback}}} becomes a default too.
• Names keep their case, so you keep sending FIRST_NAME. A name with dots is flattened: {{{order.id}}} becomes {{order_id}}.
• {{{UNSUBSCRIBE_URL}}} (and {{{RESEND_UNSUBSCRIBE_URL}}}) becomes {{{unsubscribe_url}}}, which coss.com fills on marketing sends.
• FIRST_NAME, LAST_NAME and EMAIL came from the contact on Resend's broadcasts. Here they are ordinary variables: pass them when you send.

A template whose placeholders all convert is published. Anything else is left as a draft, with what to fix listed in the report. A template keeps its Resend alias when it is a valid coss.com alias (lower-case letters, digits, - and _); otherwise the alias is made from its name.

Webhooks

Resend's email and contact events have the same names on coss.com: email.sent, email.delivered, email.delivery_delayed, email.bounced, email.complained, email.failed, email.opened, email.clicked, email.scheduled, email.suppressed, email.received, contact.created, contact.updated and contact.deleted. contact.topics.updated becomes contact.unsubscribed and contact.updated. Domain, suppression, topic and inbox events have no coss.com counterpart, and the report lists them as left out. The payloads differ from Resend's, and requests are signed with the endpoint's new secret, so update your handler before you enable an endpoint (see verifying requests).

Domains and DNS

coss.com's DKIM records don't collide with Resend's, so add them and keep sending through Resend while they verify. Both use the domain's send subdomain for bounces (its MX and SPF records): compare the send records coss.com shows with Resend's, and change any that differ when you switch your sending over.

Switch your code

coss.com's SDK returns { data, error } like Resend's and takes the same fields, in snake case (reply_to, scheduled_at):

const { data, error } = await coss.email.send({
  from: "Acme <hello@acme.com>",
  to: "ada@example.com",
  reply_to: "support@acme.com",
  subject: "Your receipt",
  html: "<p>Thanks for your order.</p>",
});

Send an imported template by its alias, with its variables:

const { data, error } = await coss.email.send({
  from: "Acme <hello@acme.com>",
  to: "ada@example.com",
  template_id: "welcome",
  variables: { FIRST_NAME: "Ada" },
});

Topic choices apply to sends that name the topic's key as their category.