Validating XML email

[Kenneth Porter <sh...@sewingwitch.com>]

I noticed some spam using XHTML, which I understand is HTML with stricter 
XML validation rules. Just out of curiosity, I ran it through the W3C 
Validator and it had quite a few errors.

<http://validator.w3.org/>

Now if someone goes to the trouble of claiming their mail is XHTML, then it 
seems the least they can do is verify that it's well-formed, even if it 
doesn't follow its DTD, and it would be better it if was valid under its 
DTD or style sheet.

So I'm wondering how much legitimate HAM that uses XHTML fails a validator? 
If one rejects XHTML or XML that is not well-formed, how much HAM will one 
turn away?