Serve images in emails from http or https?

Question

Does it make a difference at all whether I serve the images in an email campaign from an encrypted domain? Will this make emails less likely to end up in a client's spam folder?

Answer 1

Using http insteadt of https will most likely not affect the spam/ham classification.

However, it could throw warnings in older browsers, which may confuse your recipients. The reason is because most webmail services - like Gmail, Outlook.com (former Hotmail), or Yahoo! Mail - default to SSL when the user logs in. Loading images from non-secure sources now is a possible security leak. This blogpost and the screenshot below illustrate the problem and the resulting browser behaviour.

Furthermore, you'll miss referrer information of a recipient who opened an email on https, when the tracking pixel is located just on http.

Finally, if you use https, make sure the certificate is valid and up-to-date. Otherwise email clients like Thunderbird throw warnings.