What are ways to increase my Mandrill sender reputation?

There are a few things that can be done:

*1- Setup your Magento transactional email to go through Mandrill as well.*

This includes things like your order confirmation and shipping confirmation emails. You can do this by installing SMTP Pro which is a popular and free extension and then hook it up to send through Mandrill.

*2- Enable more MageMail email triggers.*

If you only have a few email triggers enabled, it's actually better that you enable more emails. There will always be the random spam complaint that you get from time to time, but Mandrill looks at the ratio of complaints to send volume. So increasing your send volume with email that customers are not complaining about will improve your sender reputation.

*3- Configure your SPF / DKIM records*

Here's how you can do that