Equinux offers a variety of professionally designed email templates (a.k.a. stationery packs) that look and work great in both Direct Mail and Apple Mail. Their professionally-designed templates include seasonal options, business-use templates, and even categories like Love & Romance. Equinux is not affiliated with e3 Software.
You can import any web page or HTML email template straight into Direct Mail. Direct Mail understands template languages for Campaign Monitor and MailChimp, so you can use their designs, too. Here are some resources for HTML email templates:
Equinux Mail Designer's interactive template editor lets you make your own designs. Templates you create with Mail Designer work not only Direct Mail, but also Apple Mail. Equinux is not affiliated with e3 Software.