Do you have any advice on how to improve deliverability?

Some useful information on improving deliverability:

VerticalResponse, Email Delivery & You: http://help.verticalresponse.com/images/uploads/Email_Delivery.pdf

Boing! Everything You Ever Wanted to Know About Why Emails Bounce: http://www.verticalresponse.com/sites/www.verticalresponse.com/files/bounce_guide.pdf

Design Your Email for Delivery and Response: http://help.verticalresponse.com/how-to/tutorial/design_your_email_for_delivery_response/

Email Delivery: Authentication, Reputation and Feedback Loops, What it All Means: http://blog.verticalresponse.com/verticalresponse_blog/2010/03/email-delivery-authentication-reputation-and-feedback-loops-what-they-all-mean.html