Massive Nonsensical 301 on Large ecommerce Site

[Bevelwise]

We are in the process of launching a large ecommerce site, which is a rebuild.  Their old URL structure does not make it possible in our eyes to logically map every URL to it's corresponding new page.  We have done our best to properly and manually redirect all pages that were receiving any amount of organic traffic and have also covered all pages that had external links.

Our question is we will end up with potentially tens of thousands of 404 errors that will never fix themselves.  The manual work will need to stop at some point.  Would it be better to leave these 404's the way they are and just let them fall out of the index or should we take everything we cannot assign appropriately to a page like the products root or the home page?

I'm also open to hearing any suggestions about how others have solved massive nonsensical 301's.

Thanks in advance,

[Chris.Menke]

With that many pages, they might not all be indexed anyway.  If they weren't "important" enough to get traffic to the old site, there's probably no real strength in them that needs to be 301'd to the new site.

[Everett]

If you can write a regular expression to redirect only the old URLs to a new page, such as the home page, or closest category page - without redirecting every mistyped URL - then I would go ahead and do that. However, you do not want to redirect every mistyped URL because that would create a "soft 404" situation.

As Chris Menke mentioned below, if you have already redirected all of the "top pages" the rest, which probably have little or no external link authority, can just go to a 404 page, and will eventually be removed from the index.

You will want to pay close attention to the 404 reports for several months afterwards just to make sure that you haven't missed any URLs with significant external traffic or links.

If you really wanted to put in the extra effort you could redirect any page that gets more than 5 visits a day/month (you choose the threshold), and any that have at least one external link. That's what I would do, personally.

Also, if the category name is in the old URL you could make the "catch all" redirects go to the most appropriate category instead of all going to the home page. It all depends on what you have to work with. If they are truly nonsensical you will probably end up just letting all of the URLs with no traffic or links 404.

Good luck!