I've found that unless you use some of the big-name e-mail providers, there's a high chance your letter wasn't even seen.

Companies tend to use some providers e-mail solutions and some of them are way too trigger-happy for spam.

It's gotten so bad, that before applying, I look at their MX records and if it's gmail, I won't even bother unless i really-really want the job. In that case I follow up with a phone-call.