I can use premailer to preview how my email will look like in an actual browser before sending it out into cyberspace! Nothing really; I honestly think this program does what its supposed too do and you shouldn't have any issues using it as long as your not trying to send images with style backgrounds or anything of such kind ;).
Give it ago once if it has helped save some time when checking emails through different browsers. It helps me check all aspects from desktop version, mobile versions etc so i dont need to download each one separately just to be able to view them at ease.