Do you have any advice on how to improve deliverability?

Some useful information on improving deliverability:

VerticalResponse, Email Delivery & You: https://verticalresponse.com/blog/the-ultimate-email-delivery-guides/

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