Pivotal Veracity recently tested hundreds of HTML e-mail messages to see whether they landed in the inbox or the bulk folder, or were blocked outright.

They came up with these surprising results:

• A tracking beacon below the closing HTML tag will get e-mail filtered to the bulk folder at MSN/Hotmail.
• A poorly constructed boundary between the text and HTML portions of a multipart e-mail message also sends the e-mail to the bulk folder at MSN/Hotmail.
• Using hex-encoded domains in URLs (substituting the code "%20" for a space in a URL, for example) can get your e-mail blocked or sent to the bulk folder at AOL, CompuServe, and MSN/Hotmail.
• Using a decoy link that shows one URL in the e-mail but actually redirects to another URL when clicked also gets e-mail directed to the bulk folder on MSN/Hotmail.

This isn't a common technique used by most legitimate e-mail marketers, but if you're thinking about doing it, they advise against it.

Using decoy URLs is a technique commonly employed by phishers, scammers who impersonate financial institutions to steal Social Security, bank, or credit account numbers.

For example:
<br /> [block]0[/block]<br />

Readers see the second URL in the message, but they'd be sent to the first URL.

via How HTML Code Affects E-Mail Deliverability