Why won't they open our emails?

So after completing a comprehensive email acquisition focused online marketing campaign you begin mailing your precious leads only to find that they aren't opening your emails. What could be the reason for this, after all you spent loads of time identifying who they are, where they go and what message would motivate them to sign up...

Well the answers could come from multiple causes
1) Email acquisition
How were the names acquired? Were they incentivized with a sweepstakes? Were they gathered via co registration? Either tactics will result in a lower quality (but lower cost) list of addresses
2) Assimilation
Were expectations set upon registration? Did the customer receive a welcome email setting the tone and reinforcing expectations? Welcoming the customer and letting them know what to expect will help them make a decision on whether to stay on the list and/or open the emails.
3) Content
Did you make good on your expectations? Is the customer recieving content truely valuable or just a string of promotions, discounts and sales?

Today's internet savvy customer demands customized/personalized (whether by behavoir or selections) material and will reward the companies who provide it and shun the ones who dont.