Hi, I uploaded our css stylesheet as such <link rel="stylesheet" href="http//www.externalcsslink..."> to the header in the html code of the email template. I also tried adding it to the 'replace html' section of the email itself. Neither renders when a certain class is called. Has anyone else encountered this problem?
It's not a "problem" -- as there's no reason to think this would work. Mail clients don't even reliably support local <style> elements, let alone remote-included styles.
In email, both techniques are not valid. All CSS classes need to be defined in the email itself and furthermore, many of the css styles will have to be inlined.
Just a bit of a footnote here: you can use external CSS spreadsheets in emails if (and only if) you want to target email clients that do, in fact, support external CSS—most commonly this is done for webfont support (as it's basically a 1:1 overlap of those who support <link> in <head> and those who support webfonts), but this is also a nice workaround for adding SVG to supported clients, advanced animations, etc.
Then again, this is pretty advanced email design stuff for the vast majority of folks.