I am looking for a bit of advice. We have recently inherited a website which has recently been re-designed and moved to a new server.
The old site was built in Coldfusion and the new site has been re-built in Wordpress and has been migrated to a new server, which has created a lot of errors in Webmaster Tools. The old site has now been deleted.
It is currently showing 3,220 404 errors.
The old site had non SEO friendly URLS, for instance:
The new site has SEO friendly URL's
So my question is what is the best way to handle all of these 404's? They are all linked from the old site (which no longer exists). I can't 301 redirect them as a, i don't know what was on the page previously (as it no longer exists) and also, a lot of them were dynamic pages.
So, can i just leave them, instead of re-directing them? If i do that, how long will they take to disappear from the SERPS?