It's not the function's fault. The filters' main criteria are the content of the email.
Try using words very specific to your business, and try not to sound like a cheap spam offer. So avoid phrases like "limited time offer" and "free bonus."
By "specific" words I mean, "Harvard" if you're associated with Harvard, or Alpha Particles if you're doing something with alpha particles.
And of course, don't mention penis enlargement =) Although if you do, you probably won't want your clients to see the email anyway...