### [MailerSend Unauthenticated](https://wpmailsmtp.com/docs/mailersend-unauthenticated/)

**Published:** April 22, 2026
**Author:** David Ozokoye

**Content:**

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

## What this Error Means

The API token in WP Mail SMTP is missing, invalid, or has been deleted from the MailerSend account. The request can’t be authenticated.

## Common Causes

- API token deleted or regenerated in MailerSend without updating WP Mail SMTP
- Typo or truncation when copying the token
- API token field left empty in WP Mail SMTP
- Token has expired (MailerSend tokens have a configurable expiration date)
- Token belongs to a different MailerSend account than the one with the verified sending domain

## Resolving the ‘Unauthenticated’ error

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

### Step 1: Generate a New API Token in MailerSend

Log in to your **[MailerSend](https://app.mailersend.com)** account area. In the left sidebar, expand **Developer tools** and click **API tokens**.

![API tokens page in MailerSend Developer tools](https://wpmailsmtp.com/wp-content/uploads/2026/04/mailersend-api-tokens-page.png)Click **Create new token**. Enter a name for the token (e.g. “WP Mail SMTP”). Leave the **Domain** set to **All** (or restrict it to a specific domain if needed). Optionally set an **Expiration date**. If you don’t set one, the token expires in 24 hours.

![Token creation form showing name, domain, and expiration settings](https://wpmailsmtp.com/wp-content/uploads/2026/04/mailersend-token-creation-form.png)Under **Permissions**, leave **Sending access** selected (this is the default). Click **Generate token**.

![Generate token button in MailerSend](https://wpmailsmtp.com/wp-content/uploads/2026/04/mailersend-generate-token-button.png)Copy the token immediately. It is only shown once and cannot be retrieved later.

![Copy the API token — it is only shown once](https://wpmailsmtp.com/wp-content/uploads/2026/04/mailersend-copy-token.png)### Step 2: Paste the New Token in WP Mail SMTP

Log in to the WordPress admin area. Then go to **WP Mail SMTP » Settings** and click the **General** tab.

![WP Mail SMTP setting page](https://wpmailsmtp.com/wp-content/uploads/2023/08/wp-mail-smtp-settings.png)Scroll to the **Primary Connection** section and select **MailerSend** as the **Mailer**.

![Select Mailersend mailer](https://wpmailsmtp.com/wp-content/uploads/2025/05/select-mailersend-mailer.png)Clear the **API Key** field and paste the token you copied.

![Paste API key](https://wpmailsmtp.com/wp-content/uploads/2025/05/paste-api-key.png)Finally, scroll down and click **Save Settings**.

![Save settings](https://wpmailsmtp.com/wp-content/uploads/2022/12/click-save-settings-button.png)### Step 3: Send a Test Email

To send a test email, go to **WP Mail SMTP » Tools**. Click the **Email Test** tab.

![Email Test tab in WP Mail SMTP Tools](https://wpmailsmtp.com/wp-content/uploads/2021/09/Email-test-tab-in-WP-Mail-SMTP.png)Enter a recipient email in the **Send To** field. Leave the **HTML** toggle ON.

Once done, click **Send Email**.

![Send Email button in the Email Test tab](https://wpmailsmtp.com/wp-content/uploads/2022/08/wp-mail-smtp-send-test-email-1.png)Look for the success message, then check your inbox to confirm the email arrived.

## Notes

- MailerSend API tokens are shown in full only once at creation. If you lose it, you must generate a new one.
- Token expiration is set at creation time (minimum 24 hours). If your token expired, generate a new one following Step 1 above.
- If the error persists after replacing the token, confirm you are logged into the same MailerSend account that owns the verified sending domain configured in WP Mail SMTP.

That’s It! You now know how to fix the ‘Unauthenticated’ error in MailerSend and get your emails sending again through WP Mail SMTP.

**Categories:** MailerSend Errors, Error Library

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