Best to embed everything into the email message itself. Remember most likely email clients will block external links that load additional resources as it might even get caught up in spam filters. Most common example you know are images that you have to explicitly allow your email client to download.
Depending on the appliance I would recommend you to find a tool that also allows you to test on different email clients. Something like: https://litmus.com/email-testing.
I've learned that there is always some email client that messes with your styling. In the list of things Front-Enders don't want to do this might even get below styling for IE 6.