Create newsletters in Letterhead while sending through Iterable or Mailchimp

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 focus of this article. Letterhead also connects to SendGrid, Constant Contact, HubSpot Marketing, Sailthru, and Cordial (with Braze coming soon) — you’ll find the full provider comparison on the Sending settings page. 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:

• Prefix (required) — your Mailchimp server prefix, visible in the URL when logged in to Mailchimp
• API Key (required)
• Reply-to email address (optional)
• Per newsletter, you select the Mailchimp audience list it should send to — Letterhead reads the lists from your account

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

Caution

Switching a channel’s ESP integration disables the templates and automations tied to the previous integration and unschedules any scheduled newsletters associated with it. Plan the switch for a quiet moment in your calendar.

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.