If you are sending email from one of the email marketing websites to your site via Postie you may notice that the formatting isn't the same. There are 2 possible reasons for this:
Your WordPress theme conflicts with the styles you assigned when building your email.
In this case you will either need to change the style of the email or the theme. You might be able to add some additional CSS via a plugin like Simple Custom CSS.
Your email does not have inline styles
If your email depends on external or internal stylesheets Postie removes these in order for the resulting post to be valid for WordPress. All the email marketing site recommend that you use inline styles for greatest email client compatibility and this works best for Postie as well.
MailChip in particular has a nice setting to automatically make all the styles inline. See http://kb.mailchimp.com/campaigns/ways-to-build/use-the-css-inliner
Constant Contact CSS recomendation: http://support2.constantcontact.com/articles/FAQ/1095
MailChip also provides a free tool for converting your HTML into inline style. Make sure you copy the entire html source then go to http://templates.mailchimp.com/resources/inline-css/