Big Email File Size, Big Problems

Email deliverability is a game of chance when your email file size is working against you. In this article, we explore what email file size actually means, how it affects your campaigns, and what you can do to keep your emails lean and landing in inboxes. Whether you're new to email marketing or you've been at it for years, file size is one of those things that quietly causes problems. The good news is that it's also one of the easiest to fix once you know what to look for.

Let's break it down first

In simple terms, email file size refers to the total size of your email message and any attachments you include. Every element in your email contributes to its overall "weight": the HTML code, images, videos, and any audio files all add up.

That weight matters more than most people realise.

What is email deliverability?

Before we get into the effects of file size, it helps to understand what email deliverability actually is. Email deliverability is the ability of an email to reach your subscribers' inboxes without being blocked, filtered, or marked as spam. Several factors affect deliverability: your email's content, your sender reputation, authentication, and yes, file size. If you want a full refresher on the topic, our guide to email deliverability covers it in detail.

How does email file size affect deliverability?

Heavy emails need more time to render. That means delays, and it means a slower experience for the person on the other end. Larger emails also take longer to load. This is especially true on mobile devices, where the majority of your subscribers are reading. If your email loads slowly, most people won't wait for it. And then there are spam filters. Emails that are overloaded with images, videos, or attachments are more likely to get caught before they even reach the inbox.

What is email clipping, and why does it matter?

Email clipping is what happens when your email's HTML content exceeds 102KB. Gmail, in particular, enforces this limit strictly. When an email is too large, Gmail hides part of the message and shows a "View entire message" link instead.

This is a problem for a few reasons:

• Your subscriber might not see your call to action
• Your footer and unsubscribe link can be hidden, which creates compliance issues
• Any content below the cut-off point is effectively invisible unless the subscriber actively clicks through

It's worth noting that clipping is based on the HTML content size of your email, not the total message size. Email providers add "overheads" (like headers and MIME structure) after sending, but these don't affect Gmail's clipping threshold. The 102KB limit applies only to your email's HTML code.

Mail Blaze warns you before you send

This is where Mail Blaze does something useful. Rather than leaving you to discover a clipping problem after a campaign has gone out, the platform flags it during the build process. There are two places where you'll see your email size displayed:

When you click Save and Next

If your email HTML exceeds the allowed size limit, a pop-up warning appears before you can proceed. This gives you the opportunity to reduce your email size before the campaign goes any further.

On the Campaign Summary page

Your approximate email size is shown again on the summary screen. If it's within a safe range, the figure appears in black. If it's at risk, the text turns orange. You'll also see a status label, either "Not at risk", "Might be at risk", or "At risk", so you know exactly where you stand. Clicking the info icon next to the size indicator shows the clipping threshold and links through to a full support article with steps on reducing your email size. For a full walkthrough of how the warning works and how to bring your email size down, visit our support article on email size and clipping.

What can you do to optimise your emails for maximum deliverability?

Keep your HTML content below 102KB

This is the threshold Gmail uses to determine whether to clip your email. Aim to stay under 100KB to give yourself a safe margin.

Share files via the Media Library

Mail Blaze doesn't support file attachments in campaigns. Instead, upload your files to the Mail Blaze Media Library and link to them via a button or hyperlink. This is actually better for your subscribers too: one click takes them straight to the file, and it has no impact on your email size. The Media Library has a 2MB limit per file. Here's how to set it up:

• In the email builder, go to the Content tab and drag a Button element into your preview
• Click the button to open the editing panel
• Update the button text to something relevant, for example "Download Brochure"
• Click Choose File to upload your document to the Media Library
• Once uploaded, hover over the file until you see Insert, then click it to attach the link to your button

Your subscribers can access the file with a single click.

Compress your images

Images are one of the biggest contributors to email weight. Compress them before uploading. JPEG works well for photographs, while PNG is better suited to logos, icons, and images with transparent backgrounds. Where possible, reduce image dimensions to match how they'll actually display in the email rather than uploading oversized files. Limit desktop-only or mobile-only blocks

When you add a block set to display on one device only, the code for that block is still included in every send. If you're close to the size limit, reducing the number of device-specific blocks can make a meaningful difference. Avoid embedding too many images

Embedding images directly in your email increases its size significantly. Where you can, host images externally and reference them via URL rather than embedding them.

Key takeaways

The size of your email has a real impact on whether it reaches your subscribers, how it renders, and whether the full message is even visible. Keeping your HTML content under 102KB, using the Mail Blaze file manager for attachments, and compressing your images will go a long way. And if you're ever unsure whether your email is at risk before you send, Mail Blaze will tell you. The size indicator is there at every stage of the process, so you can fix the problem before it becomes one.