I'm wondering how the generation of email previews takes place in those applications which test emails across OS/desktop applications/web services. Note that the issue is previews of email, and not websites which browsershots.org is an example of (although their result is somewhat like what I want to achieve).

nonprofitmarketingguide.com/blog/2009/01/06/see-what-your-e-newsletter-looks-like-in-different-email-programs/ describes this kind of service and mentions the companies Mailchimp and Campaign Monitor as examples. I assume they developed their own solutions, but is there (somewhere deep down) a common ground that their creations originate from?

About the choice of forum, this would be appropriate from a testing point of view; but feel free to move this topic to wherever the chance of good answers is higher.