I recommend that people not use such email templates: instead, just produce simple, unvarnished content that roughly matches what you’d produce if you just wrote a normal email manually in your normal email client.
Note that I’m not saying “use text/plain and no text/html part” as some will; you’re welcome to use HTML (though I do say, please provide a good text/plain part), but see what life’s like if you roughly limit yourself to paragraphs, lists, inline formatting, images (where useful, not just gratuitous) and maybe headings and tables (strictly for data, not for layout; receipts often benefit from tables). And maybe a company or product logo to go with it, but not formatted fancily. Maybe go a teeny bit fancy for a big call-to-action button—or perhaps just try having it as a bold link in a paragraph of its own.
Quite a few entities that have tried this have reported higher conversion rates and customer satisfaction.