[Samuel_Michon]

Use the CSS Inliner Tool:

http://beaker.mailchimp.com/inline-css

Also, read MailChimp's study on responsive emails:

http://mailchimp.com/resources/guides/html/email-on-mobile-d...

[nnnnnn]

To add to Sam's comment, if you're using Rails, try:

https://github.com/fphilipe/premailer-rails3

There are a few gotcha's with the gem but overall it works pretty smoothly!