I submitted a sitemap with incorrect URLs. After a few days I looked at Webmaster Tools crawl errors and I saw 1000's of URLs with not found errors.
So I went and fixed the problem with the URLs and resubmitted the sitemap.
The problem is that Webmaster Tools is still showing me the same old not found errors.
1) I know that the URLs in the updated sitemap are ok
2) I can see that the date of the not found error URLs corresponds to when the problem was.
Now I tried using the "mark as fixed button" - but that only lets me select the first 1000 URLs, and then it shows an empty list.... and I have about 40,000 not found URL errors.