1. Install Post SMTP in version <= 2.7.0 and configure it.
2. Send email using any contact form which uses 'wp_mail' function. Include the following payload in the message:

<img src=x onerror=alert(document.domain)>

3. Visit /wp-admin/admin.php?page=postman_email_log (Post SMTP -> Email Log)
4. Click 'View' next to the first record.
5. The message is shown and JavaScript code is executed.