I haven't delved into custom filters, so don't know what they're capable of. In principle, do the filter rules have what it takes to prevent an email client from downloading tracking pixels in html emails? The kind of rules I'd have in mind might be something like: disallow download of images from urls containing 'track' after the hostname disallow IMG tags with width & height of 1px They wouldn't have to be perfect -- certainly a few false positives would be fine. And I can write the rules myself. But I'd like to know if it's possible before poring over the docs.