I'm jumping in at the end here so I hope I'm not being redundant. It's true that the HTML Editor control we provide within F1 had (may still have) a few issues with special characters. However, formatting and layout of HTML emails within a variety of email clients like Yahoo, Hotmail, Entouch, etc., etc., etc. will invariably offer different results and has nothing to do with Fellowship One. You will never get it to look exactly the same. We've worked in the past with a company that specializes in nothing but email campaigns, the company is CampainMonitor.com. Their sole focus is email and I would consider them experts in the field. They have many, many articles on best practices for properly designing HTML emails but even they readily admit they'll never be uniform. They even highlight that Microsoft's latest release of Office 2007 was a step backward in support of standards.
Here's a link to one of the articles highlighting a difference with how images are handled.
Curtis Simmons
VP, Product & Services