> ## Documentation Index
> Fetch the complete documentation index at: https://loops.so/docs/llms.txt
> Use this file to discover all available pages before exploring further.
# Update an email message
> Update fields on an email message.
Content API endpoints are currently in an open alpha and are subject to
change.
## Request
The campaign that owns this email message must be in `Draft` status.
### Path parameters
The ID of the email message.
### Body
The `contentRevisionId` you last fetched, or the `emailMessageContentRevisionId` you received when creating the campaign. Used for optimistic concurrency.
Email subject line.
Email preview text.
Sender display name.
The sender username (without `@` or domain). The team's sending domain is
appended automatically.
Reply-to email. Must be empty or a valid email address.
The email body serialized as [LMX](/creating-emails/lmx). Styles must be embedded in the LMX
`` tag. The LMX payload must not exceed 100KB.
## Response
### Success
The current content revision ID. Use this value as `expectedRevisionId` on
your next update request.
Non-fatal issues raised while compiling the submitted LMX. Only present on update responses when warnings were produced.
Always `"warning"`.
Optional path for the warning location.
### Error
If the request body is invalid, a `400 Bad Request` is returned.
A `404 Not Found` is returned if the email message does not exist.
A `409 Conflict` is returned when the campaign is not in draft status, when
`contentRevisionId` is stale, when content cannot be parsed, or when the email message uses MJML format.
A `413 Payload Too Large` is returned when the LMX payload exceeds the 100KB limit.
A `422 Unprocessable Entity` is returned when LMX compilation fails.
If the API key is invalid (or content API is not enabled for your team), a
`401 Unauthorized` is returned.
```json Response theme={"dark"}
{
"success": true,
"emailMessageId": "em_01hxyz",
"campaignId": "cmp_01hxyz",
"subject": "Big spring updates",
"previewText": "A quick look at what's new",
"fromName": "Loops",
"fromEmail": "hello@news.example.com",
"replyToEmail": "support@example.com",
"lmx": "Hello world",
"contentRevisionId": "rev_01hyza",
"updatedAt": "2026-03-28T15:30:00.000Z"
}
```
```json Error response theme={"dark"}
{
"success": false,
"message": "An error message."
}
```