You've devised a brilliant opt-in marketing campaign, complete with an educational newsletter that's all the rage. Your sales are spiking and your list is growing.

Then it happens -- some spammer uses phraseology that is similar to your corporate name or some malicious code writer launches a virus with a common word in your product description. Suddenly your permission-based e-mail is blocked by half the ISPs on the planet.

Frustrating, to say the least. But you are not alone.

The top ISPs blocked 22 percent of permission-based e-mail in 2004, according to a study by e-mail services provider Return Path. In fact, corporate e-mail considered spam by ISPs is a growing problem, up 3.3 percentage points over the second half of 2003.