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I don't think SEOmoz has a tool like SEMRush, although I think it would be a great idea if SEOmoz did create a similar tool. They already have a huge index with OSE so I think it would be a natural extension using their existing data. Or they could just buy SEMRush.

Download Screaming Frog spider and crawl your website. That should help you find if/where you reference 404 pages. Cheers