Html emails require a mixture of tables (an otherwise largely obsolete layout method, now replaced by css), remote images and inline css styles.
Many people misunderstand the correct way to design and code a cross-client compatible Html email, and ensuring across the board support for all your recipients can be a detailed task!

Precise markup and testing is required across Thunderbird, Outlook, Outlook Express, Apple Mail, Gmail, Hotmail, Yahoo, AOL etc. Hotmail also requires some specific css hacks (such as adding a style="display: block;" to images). Campaign Monitor provides this excellent Guide to CSS support in email clients.

I have started using Email on Acid (free beta) and Litmus (subscription) as a quick snapshot and backup to physical testing. Although these browsercam-style services are mostly accurate, I have discovered that this is not always the case, so I don't rely too heavily on them.

Some potential obstacles for designing Html emails:

• AOL, Outlook 2003, Outlook Express and Gmail turn off images by default. This means you should provide a text-only version or web page version of the email. Also use alt tags and specify width and height for every image to make sure some content is displayed within the correct layout even when images are switched off.
• Background images have limited support and where the design allows, should be avoided. repeating table background images can be used, as can solid colours

I also provide mailshot and list management services running on PhpList or Groupmail.
These allow the following features:

• double opt-in subscription mechanism
• scheduling
• RSS
• list segmentation
• click-tracking
• attachments
• bounce management

I can design the emails according to your brand guidelines, or consult with your designer on best practices to build a robust, lightweight and user-friendly Html email.