This article explains how Letterhead’s External Sending Integrations work for both Iterable and Mailchimp, why they exist, and what limitations or requirements apply when using either platform as your sending provider.
What Is This Integration?
The External Sending Integration allows you to create and edit newsletters inside Letterhead while sending them through Iterable or Mailchimp as your Email Service Provider (ESP).
This means you can use Letterhead’s newsletter creation tools without needing to migrate:
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Your audience
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Your sending infrastructure
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Your existing Iterable or Mailchimp workflows
In this setup, Letterhead acts as the editor, while Iterable or Mailchimp remains the sender.
Supported sending providers
Letterhead currently supports the following external ESPs:
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Iterable
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Mailchimp
1. Sender Setup
To send newsletters using an external provider, you must configure the sender at the organization or channel level.
Iterable setup requirements
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Iterable API Key (required)
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Sender selection: Iterable
Mailchimp setup requirements
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Mailchimp Prefix (required)
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API Key (required)
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List ID (required)
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Reply-to email address (optional)
Once the API Key field is completed, users can save button and begin using Letterhead as their editor.
What happens after connecting:
- Iterable or Mailchimp becomes the active sender for newsletters in that channel
- Letterhead routes newsletter drafts to the selected ESP for delivery
- Sending, audience handling, and deliverability are managed by the external provider
2. Newsletter Creation & Previews
You can still use the full Letterhead editor:
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Drag-and-drop content blocks
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Custom templates
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Saved snippets (where applicable)
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Preview and test your design directly inside Letterhead
Preview and design functionality remains available even though the newsletter is sent through an external ESP.
3. Audience management behavior
When an external sender (Iterable or Mailchimp) is selected:
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The Audience tab in Letterhead is hidden
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Subscriber data remains fully managed in the external ESP
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Letterhead does not store or modify audience records
All audience updates must be made directly in Iterable or Mailchimp.
4. Metrics and reporting
When using an external sender, Letterhead captures basic engagement metrics, including:
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Opens
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Clicks
For more advanced analytics, reporting, or audience-level insights, refer to your ESP’s native reporting tools.
5. Features not available with external sending
When Iterable or Mailchimp is selected as your sender, the following Letterhead features are not available:
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Audience management
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A/B testing
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UTM code configuration
These features rely on Letterhead’s native sending system and do not function with external ESP-based sending.
What should I know before enabling this?
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You must have an active Iterable or Mailchimp account
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API credentials must be generated in the external platform
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Audience changes must always be made in the ESP
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Metrics visible in Letterhead may be more limited than ESP-native reports
Is external sending right for me?
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Use Letterhead native sending if you want full access to audience tools, A/B testing, and advanced analytics inside Letterhead
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Use external sending (Iterable or Mailchimp) if you already rely on an established ESP and want to keep your existing setup while designing newsletters in Letterhead
Need help?
If you have questions about setting up an external sender or need help troubleshooting an integration, contact support@tryletterhead.com or submit a ticket through the Help Center.