I mentioned abit about this on my adCenter Blog post last week:
"Why are my ads/KWs being rejected?
To create a great user experience for Live Search we need to try and make sure that when people search for something, the ads on the results page are relevant, and when they click through to your website, they find what they’ve been looking for.
We have systems in place that check for relevance. Sometimes KWs/ads are rejected because of inappropriate content or because they do not adhere to our editorial guidelines.
If any KWs or ads are rejected you’ll be sent an e-mail letting you know and pointing you to our Editorial Guidelines. If you feel they really should have been accepted, please contact Microsoft adCenter Support and ask for an editorial appeal template. Fill it out and send it back and we’ll have one of our editors take a look. The process should take around five days to complete."
I'm concerned you see a 404 Error from links in the rejection email:
Do check these links out and if necessary submit an appeal. Also please let me know if you're still seeing the 404 behaviour.