Emails going thru our spam filters are stripped of I-Frames for security purposes.  This may affect some emails that attempt to put a form for interaction.
A quick lesson, if interested. (otherwise no reason to continue)
Some malicious emails contain 'I-Frames' in the html of the message.  I-Frames allow a web page to include content from a complete different website on the Internet, usually scripts.  This could allow an email to skip past scanners (since it doesn't have any dangerous scripts), and then when it's viewed it calls the script from that malicious site.

Of course, there are plenty of legitimate reason for i-frames, and they are usually not malicious, but they are not needed in emails.
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From the Web:
An embedded iframe in email message could be used to run some VB script, and this script could have access to the local file system. For instance, it could read or delete files.

Besides that simple attack, in conjunction with email attachments with forged Content-Type and Content-ID headers, iframe allows to run any executable attachment, this is called MIME header attack. Klez, for example, uses this trick.  CLSID attack (forged file name ending in CLSID) also is used with an iframe.

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http://www.theregister.co.uk/2004/07/30/email_privacy/
They Like to Watch

ReadNotify makes Jeremy Bentham's dreams of surveillance look mild. They allow users of an almost every major email program in use today - Outlook, Outlook Express, Netscape Mail, Eudora, Thunderbird, Pegasus, even Hotmail and Yahoo - to create email that gives the sender an enormous amount of information about the recipient, as they describe on their site (again, no direct link to these guys):