Mandrill is a service by Mailchimp for reliable transactional email sending. Whether you use Shopify, WooCommerce, Bigcommerce or even WordPress you should be using a service such as Mandrill to ensure that your emails have the best chance of being received. I get lots of people that use all sorts of web software (particularly WordPress) that have email delivery issues until I connect them up to Mandrill.
For the best chance of recipients receiving your email you should always setup DKIM and SPF records on your sending domain. Mandrill is no different and provides instructions on how to setup these DNS records. An issue I’ve noticed is that even when I’ve setup the necessary SPF record in my DNS host (CloudFlare), Mandrill still notes ‘SPF missing’.
In short, if you’ve setup your SPF record for Mandrill correctly and receive errors then add the SPF as a TXT record instead. This works for CloudFlare and should word for GoDaddy and other DNS hosts.