Subject Line Testing API

No more manual approval required! Unlike other services that require you to email for custom endpoints, our system is fully automated.

Simply register an account, add your domain and/or IP address to the whitelist in your dashboard, and start using the same API endpoint as everyone else. No waiting, no custom endpoints, no hassle.

Add the following to your site alongside any existing JS includes. Unlike competitors, everyone uses the same endpoint:

It's likely that you want the Subject Line Tester to be embedded as part of an existing form within your application. Add eslt-field as an additional class on the fields you'd like the score to display next to:

EXAMPLE

By default, the script uses our production endpoint. If you need to customize the API URL (for testing or custom deployments), you can configure it:

<script>
// Optional: Configure custom API URL
window.ESLT_API_URL = 'https://emailsubjectlinetester.com/api/analyze';
</script>

The script will automatically score the current values of eslt-field input fields and display scores. As users type, the scores update automatically (throttled to one update per 350ms).

CSS is automatically included! Unlike competitors who require you to add CSS, our script automatically injects all necessary styles for both the inline scores and full report modals. However, you can override them if needed:

/* Embedded Subject Line Score */
.eslt-preview { position: relative; }
.eslt-preview .eslt-badge { 
    position: absolute; 
    margin-left: 10px; 
}
.eslt-preview .eslt-score {
    position: absolute;
    margin-left: 46px;
    margin-top: 6px;
}

If you're using Bootstrap, we recommend this as a starting point instead:

/* Bootstrap Embedded Subject Line Score */
.eslt-preview { float: right; margin-top: -30px; }
.eslt-preview .eslt-badge { 
    position: absolute; 
    margin-left: 10px; 
}
.eslt-preview .eslt-score { 
    position: absolute; 
    margin-left: 46px; 
    margin-top: 6px; 
    font-size: 16px; 
}

/* Modal Content Customization - Card Layout */
.eslt-modal-content #subject-tester-report {
    /* Main report container */
    padding: 2rem;
    border-radius: 1rem;
}

.eslt-modal-content .eslt-badge.eslt-grade {
    /* Grade badge (A+, B-, etc.) */
    background-color: #your-brand-color !important;
    border-radius: 0.75rem;
}

.eslt-modal-content .eslt-score {
    /* Overall score display */
    color: #your-brand-color !important;
}

.eslt-modal-content .eslt-overall-progress {
    /* Overall score progress bar */
    background-color: #your-brand-color !important;
}

/* Individual Metric Cards */
.eslt-modal-content .border.rounded-lg {
    /* Metric cards container */
    background-color: #your-light-color;
    border-color: #your-border-color;
    padding: 1.5rem;
}

.eslt-modal-content .font-semibold.text-sm {
    /* Metric names */
    color: #your-text-color;
}

.eslt-modal-content .text-xs.text-gray-600 {
    /* Metric descriptions */
    color: #your-description-color;
}

/* Individual Progress Bars */
.eslt-modal-content .h-2.rounded-full {
    /* Individual metric progress bars */
    /* Colors are set via inline styles: */
    /* Green: #10b981, Yellow: #eab308, Red: #ef4444 */
}