I have at the start of the HTML Template:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org
/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta name="viewport" content="width=300, initial-scale=1.0, target-densitydpi=device-dpi">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="format-detection" content="telephone=no" /> <!-- disable auto telephone linking in iOS -->
<head>....
However, when it comes to old email clients, my HTML doesnt render at all, and is blank... I have tested this on litmus.
Is there something that I need to add or remove from above so that it also renders in the old browsers as well.
Thanks
Your meta tags should be inside the <head>.
What you have there otherwise looks fine, so it may have something to do with the rest of your document. I would recommend starting with Sean Powell's email boilerplate and bringing some of your code over.
Failing that, have you forgotten to close any elements such as tables/rows/cells?
Which email clients is it broken in?