I use Outlook 2003, and I have Word 2003 set as the email editor.
I made a simple signature file, which contains only "<marquee>test</marquee>", and I saved it in the signature directory.
When I try to send an email to myself using that signature, when I receive the email I don't see any text and when I select the "View Source" option, it looks as if the <marquee> tag and everything in it got stripped from the email.
If I use the bulit-in email editor instead of Word the signature works fine, and the recipient sees it fine.
The stange thing is, if I send the email to myself using the built-in email editor, I can switch back to using Word as the email editor and reply to the message I sent myself, and the original <marquee> tag is still there and works like it should.
I've tried this on two different computers, to be sure it is not just some strange configuration problem.