WordPress now supports WebP images, and this plugins webp-uploads module explores using WebP as the default sub size image format. During testing with WebP, one issue raised was support for the WebP format in email clients.

Using WebP images in emails

According to Can I Email (at the time of this writing) support is very good, with >97% supporting either directly or by converting WebP images to JPEGs. However, one important exception is Outlook on Windows which does not support WebP.

Because WordPress sites can use their media library as the source of images when sending emails, for example newsletters - see https://wordpress.org/plugins/search/newsletters/ - we need to ensure we examine this issue.

Questions:

• Where do top newsletter plugins get their image URLs from? If RSS feed for example, can we keep the original (JPEG) images there in all cases?
• Is there a way to create emails that fall back to the original for non supporting clients (eg picture element)?
• How many Outlook on windows clients still vs outlook.com? caniemail suggests <3% so maybe sites can opt out if needed?
• Should we consider generating an additional medium-large image in the original (JPEG) format for use by email clients when the default output is changed to WebP so they don't have to use the original image?