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.

Answer 2

It doesn't make a difference. Almost everyone uses http:// without any spam hit, so there is nowhere for https:// to improve on that, even though spam filters are an accumulative score.

If there was something in either your content or reputation flagging you as spam, having https:// linked images wouldn't award you any 'bonus points' to save the day.

Answer 3

I second @lukeA's answer and would like to stress that the impact of using images served over http:// connections has certainly increased, even in web-based email clients.

See for example Google Inbox in Chrome (48.0.2564.109), before and after looking at an email which includes an image served over http:// (in that case, a spam email). Note the green lock is gone until you refresh the page!

Gmail's image proxy serves all images over https:// - however, it is unclear (to me) for which images the proxy is really used. For that particular spam mail that I opened to create the below snapshots, it certainly isn't.