What's Causing This Error
You'll get this error due to the below reasons:
- Make a section for styles. It's supported by Apple Mail.app, but not by Gmail or Hotmail.
- Have background image or positioning. Gmail is at blame in this case as well.
- When there are margins. Hotmail, in fact, dismisses margins. Basically, any CSS positioning is useless.
- Font-anything. Anything you try to specify using fonts is likely to be ignored by Eudora.
Here's How To Resolve It
What you can do to get over this is inline styling. Because we're just creating a one-off email, inline styles aren't quite as bad as they are on a website.