### [Error: Invalid Sender API Key](https://wpmailsmtp.com/docs/sendlayer-invalid-sender-api-key/)

**Published:** May 25, 2026
**Author:** David Ozokoye

**Content:**

**Note**: This guide applies to the [SendLayer mailer](https://wpmailsmtp.com/docs/how-to-set-up-the-sendlayer-mailer-in-wp-mail-smtp/ "SendLayer") in WP Mail SMTP.

## What This Error Means

The API key stored in WP Mail SMTP doesn’t match any active API key in the SendLayer account. SendLayer rejects the request before any email is sent.

## Common Causes

- The API key was deleted or regenerated in SendLayer without updating WP Mail SMTP
- Typo or extra whitespace when copying the key
- Wrong API key pasted (from a different account or an old key)
- SendLayer account was reset or the key was revoked

## How to Fix

Here is the complete step-by-step guide to resolving the error.

### Step 1: Copy the API Key From SendLayer

Start by logging in to your [SendLayer](https://app.sendlayer.com "SendLayer") account. Once you’re logged in, click **Settings** in the sidebar.

![Click Settings in the sidebar](https://wpmailsmtp.com/wp-content/uploads/2026/05/settings-click-sidebar.png)Then, click the **API Keys** tab.

![Click the API Keys tab](https://wpmailsmtp.com/wp-content/uploads/2026/05/settings-click-api-keys-tab.png)Finally, find the API key and click the **Copy** icon next to it.

![Click the Copy API Key button](https://wpmailsmtp.com/wp-content/uploads/2026/05/api-keys-click-copy.png)### Step 2: Paste the Key in WP Mail SMTP

After retrieving your API key, log in to your WordPress admin area and navigate to **WP Mail SMTP » Settings**. Then, scroll to the mailer section and ensure **SendLayer** is selected.

![Select SendLayer mailer](https://wpmailsmtp.com/wp-content/uploads/2026/05/sendlayer-mailer.png)Next, clear the **API Key** field and paste the key you copied from SendLayer.

![Paste the API key into the API Key field](https://wpmailsmtp.com/wp-content/uploads/2026/05/paste-api-key.png)Finally, click **Save Settings**.

### Step 3: Send a Test Email

To test the fix, log in to your WordPress admin area and go to **WP Mail SMTP » Tools**, click the **Email Test** tab, enter a recipient email in the **Send To** field, leave the **HTML** toggle ON, and click **Send Email**.

![Test email tab](https://wpmailsmtp.com/wp-content/uploads/2024/10/test-email-tab-1024x1002.png)Once done, look for the success message and confirm the email arrived in your inbox.

## Notes

- SendLayer API keys are long alphanumeric strings. Always use the **Copy** icon to avoid truncation or extra whitespace.
- If no API key appears on the **Settings » API Keys** page, the domain isn’t fully authorized yet. Complete domain verification first, and SendLayer will generate the default API key automatically.
- If the error persists after replacing the key, confirm you’re logged into the same SendLayer account that owns the domain configured in WP Mail SMTP.

That’s It! You now know how to fix the ‘Invalid Sender API Key’ error in SendLayer and get your emails sending again through WP Mail SMTP.

**Categories:** SendLayer Errors, Error Library

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