When sending an HTML email can you link to an external style sheet or does the CSS need to be included in the email?
Also, if you can link to an external style sheet, which way is more efficient/is smarter to use: linking to the style sheet or including it in the HTML?
Can you? Yes.
Should you? No. Most likely the email client will block external links and therefore your mail will not appear as you think it should.
If you must send html mail, embed everything into the email message itself. External links which automatically call back to the home server to load additional resources are one thing spam filters don't like.