When you add your website to Google Webmaster Tools, you can access various information about Google's crawling, search queries, and detected issues. Recently, I encountered a problem with a client where Google kept encountering 404 error messages during the crawling process. The 404 error indicates that the page could not be found. In this case, Google was searching for existing links, but an email address was mistakenly attached to them, rendering the link invalid. Fortunately, the solution to the problem was quickly identified.
The website had an email address displayed on all pages, but the linking was incorrect. Instead of linking to "mailto:mail@example.com," the link erroneously pointed to "mail@example.com" without the "mailto:" prefix. This caused Google to interpret it as a relative URL, transforming "http://www.example.com/blog/" into "http://www.example.com/blog/mail@example.com." Since this URL didn't exist, the 404 errors occurred.
After correcting the issue, no new 404 errors were reported the following day, indicating that the problem had been resolved.
Another potential issue is using links without a leading slash ("/"), causing Google to append the link to the current URL. This can lead to the same problem described above with the missing "mailto:" entry.