Email Deliverability: How Images Affect Inbox Placement

Email deliverability depends on many factors, and image usage plays a significant role. According to research by Litmus, many email clients still block images by default, making optimization essential for both engagement and inbox placement.

How Email Providers Evaluate Images

Spam filters consider several image-related factors:

• Image-to-text ratio: Too many images relative to text raises flags
• File size: Large images slow loading and may trigger filters
• External hosting: Where images are hosted matters
• Image blocking: Emails must work when images don't load

The Image-to-Text Ratio

The 60/40 Rule

Best practice: Aim for approximately 60% text and 40% images. This means:

• Don't send image-only emails
• Include substantial text content
• Balance visual appeal with readable content

Why It Matters

Image-heavy emails are associated with spam because:

• Spammers use images to hide text from filters
• Image-only emails provide poor user experience when images don't load
• Text allows ISPs to understand email content

Image Optimization Best Practices

File Size

Industry best practices, as documented by Email on Acid, recommend keeping individual images under 100KB for optimal deliverability and rendering speed:

• Individual images: Under 100KB each
• Total email size: Under 500KB including all images
• GIFs: Keep under 200KB for countdown timers

Image Formats

• JPEG: Best for photos, smaller file sizes
• PNG: Best for graphics with transparency
• GIF: Required for animation (countdown timers). Consider using transparent background timers for seamless integration

Dimensions

• Max width: 600-700px for email compatibility
• Use responsive sizing for mobile
• Countdown timers: 300-500px wide optimal

Alt Text: Critical for Deliverability

Alt text serves multiple purposes:

• Accessibility: Screen readers use alt text
• Image blocking: Displays when images don't load
• Spam filtering: Provides context to filters

Alt Text Best Practices

• Describe what the image shows
• Keep it concise (under 125 characters)
• Include keywords naturally
• For countdown timers: "Sale ends in 24 hours"

External Image Hosting

Why Hosting Matters

Images hosted on reputable servers improve deliverability. Note that major providers like Gmail route all images through their own image proxy servers, which cache and re-serve images to protect user privacy:

• Known, trusted domains are safer
• Fast loading improves user experience
• SSL (https) is required by most email clients

Hosting Options

• ESP hosting: Mailchimp, HubSpot, etc. host images
• CDN: Fast, reliable delivery
• Dedicated image services: Countdown timer providers like CountHub

Countdown Timers and Deliverability

When designed properly, countdown timer GIFs are generally deliverability-safe because:

• They're hosted on dedicated, reputable servers
• File sizes are optimized for email
• They add engagement value that improves metrics

Timer Best Practices

• Use a single countdown timer per email
• Include text backup describing the deadline
• Ensure the timer provider uses HTTPS
• Choose a provider with good server reputation

Testing Deliverability

Pre-Send Checks

• Test with image blocking enabled
• Check total email size
• Verify all images load via HTTPS
• Validate alt text is present

Tools

• Litmus: Email testing across clients
• Mail Tester: Spam score checking
• GlockApps: Deliverability testing

Common Deliverability Mistakes

1. Image-only emails: Always include text
2. Missing alt text: Every image needs it
3. Huge file sizes: Compress images
4. HTTP image links: Use HTTPS only
5. Too many images: Balance with text