Fortunately for the spammers, the ratio of html mail clients to non-html mail clients increases every year. Sure there's always going to be a percentage of people using clients that will not play their tracking games, but I doubt they really care about that, as there's enough non-savvy internet users for them to target, "Hey cool, I can send pretty backgrounds and colors in my emails!"
And many corporations' IT departments mandate html capable clients [cough]Outlook[/cough] anyway.

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