When you have finished coding your email it is recommended that you inlined all of your styles. Some email clients still strip CSS from the head of an email, so by inlining styles we preserve the code.
There are tools to automate this process. We recommend using Mailchimps' CSS Inliner Tool to do this.
Use the code generated here for you final email. Do not overwrite the code you have made with this, as you won't be able to continue editing. Only inline your styles when the email is ready to test and send and keep a copy of your original code so you can continue working on it in the future.