It means it will look good in Thunderbird and Gmail for Android. Thunderbird will render custom CSS it won't load any external resources though (e.g. images). M

It means it will look good in Thunderbird and Gmail for Android. Thunderbird will render custom CSS it won't load any external resources though (e.g. images). Maybe it's a nice idea to add preview to this tool how would it look like on Thunderbird and mobile.