Thanks Jason - that's a good resource.
The issue I'm facing though is on the Marketo side, not the client side. Designer sends an HTML template with
style='padding:value'
Marketo permits it in the template, but when creating an email from the template, it throws an error and strips it out, so all the padding is gone and the template is thrown off.
It seems like I'll have to have the designer re-work the template to use style='padding-left:value' and padding-top, padding-bottom, padding-right, etc.