Q: Do larger image files impact the deliverability of resort campaigns?

If you’ve spent any time in the email marketing field you’ve likely heard some form of best practice relating to limiting the size of images in your templates. But as bandwidth and mobile usage increases, does this (still) hold true? Here’s what we found.

The Goods
To find our answer we looked at 1,200 resort email campaigns sent to over 70,000,000 recipients over the last 9 months. Because this theory has been based on both the maximum file size of any one image as well as the combined file size of all images in a message, we analyzed both concepts using open rates as an indicator of success in reaching the inbox.

Let’s start with the combined file size:

And then maximum size of any one image (note the adjusted scale on the x-axis):

For our sample, open rate peaked when combined file sizes were between 100k and 200k while above and below that saw lower performance. On the maximum file size, open rates rose steadily from images between 0-50k up until 100k-250k where they plateaued for images larger that 250k.

What This Means
In terms of combined file size, it does appear that spam filters are triggered once you reach a certain level. But why does it start so low for the group that, according to some theories, should be the safest from spam? Simply put, we don’t know.

But the maximum file size chart strengthens the case that some of the things we often assume about image file sizes and spam may not be as strict as we thought.

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