Create newsletters in Letterhead while sending through your own ESP

External sending lets you create and edit newsletters in Letterhead while sending them through Iterable or Mailchimp as your Email Service Provider (ESP). Letterhead acts as the editor; your ESP remains the sender — no need to migrate your audience, sending infrastructure, or existing workflows.

Iterable and Mailchimp are the most fully supported external providers, and the ones this article walks through step by step. Letterhead connects to a range of others as well — including SendGrid, Constant Contact, and Sailthru. The Sending settings page always shows the providers currently available to your account, along with a comparison of what each one supports, so check there rather than relying on a list in this article. You can connect more than one provider at a time and assign different connections to different newsletters.

1. Sender setup

Go to Settings → Sending at the company level and choose Add a New Connection. Connecting a provider is typically an admin task. Every connection walks through the same steps: configure the provider’s credentials, select the Letterhead newsletters that will use the connection, then select the audience list for each newsletter.

Iterable configuration:

• API Key (required) and API Region
• Sender Name and Reply Address
• From Address (optional) — must be on a domain verified in Iterable; leave it blank to use the reply address. This lets the sender address differ from your reply address — for example, a brand or no-reply address while replies go to a monitored inbox.
• Additional fields for deeper Iterable setups: Subscription Channel, Unsubscribe Tag, and Data Feed IDs
• Per newsletter, you also select a Message Type ID

Mailchimp configuration — all four fields are required, and you can’t save the connection until each one is filled in:

• Prefix — your Mailchimp server prefix, visible in the URL when logged in to Mailchimp
• API Key
• List Id — the Mailchimp audience this connection works with. It’s the list Letterhead reads your contacts and engagement from, so choose the one your newsletter subscribers live in.
• Reply-to email address
• Per newsletter, you then choose which parts of that audience it sends to. The options are the saved segments and interest groups inside the list you entered above — so set those up in Mailchimp first if you want to send to anything narrower than the whole audience. You can also apply the same selection to every newsletter at once.

When you come back to edit a saved Mailchimp connection, the API key is masked and can’t be changed in place — the other fields stay editable.

This is the only place Mailchimp is set up. The same connection is what pulls your Mailchimp contacts into Letterhead, so there’s nothing separate to configure for that — see Company-Level CRM Audience Sync for what that sync does once the connection exists.

Once the connection is saved, Letterhead routes those newsletters to the selected ESP for delivery. Sending and deliverability are managed by the external provider.

Reassigning a channel from one company-level connection to another doesn’t disable anything — it just switches which connection sends the channel’s newsletters going forward. Existing templates and automations stay intact, and any newsletter you’ve already scheduled keeps its scheduled time.

2. Newsletter creation and previews

The full Letterhead editor still works: drag-and-drop blocks, custom templates, saved blocks, and previews and test sends — all inside Letterhead, even though the email sends through your ESP. If you have multiple providers connected, you can choose which provider a template sends through.

If you send through Iterable, you can also connect Iterable data feeds for per-recipient dynamic content — including personalized images — and connect data feeds at the channel level so every newsletter in the channel has access.

3. Audience management behavior

With an external sender selected:

• the channel’s Audience area in Letterhead is hidden;
• subscriber data stays managed in your ESP — make all audience updates directly in Iterable or Mailchimp;
• each newsletter sends to the audience list (and, for Iterable, message type) you assigned in the connection settings.

Letterhead still reads engagement signals from Mailchimp and Iterable to power its segment and cohort features at the company level.

4. Metrics and reporting

For newsletters sent through Iterable or Mailchimp, Letterhead captures per-send engagement metrics — opens, clicks, bounces, and unsubscribes — plus a per-link click breakdown, refreshed on an hourly sync. Delivered/accepted delivery metrics are only available for Letterhead-native sending, and for anything deeper — audience-level analytics, deliverability reporting — use your ESP’s native reporting.

5. Features not available with external sending

These rely on Letterhead’s native sending system and don’t function with external ESPs:

• Channel audience management in Letterhead
• A/B testing on subject lines
• Delivered/accepted delivery metrics

Is external sending right for me?

• Use Letterhead native sending for full access to audience tools, A/B testing, and delivery metrics inside Letterhead.
• Use external sending if you already rely on an established ESP and want to keep that setup while designing newsletters in Letterhead.

For everything the editor can do, see Working in Emails.