The snippet preview on SO displays the lowercase version, so maybe there's an additional stylesheet being loaded on your end?

Two things,

1. Create another <a> tag and check if its text is capitalized as well (Also open your developer tools and look at the applied styles on the email <a> tag)
2. Place !important after your text-transform: nones, it's likely something more specific is taking priority, and using CSS's !important would supersede that hierarchy.