High external linking issue means that a specific URL contains numerous links to third-part websites.
The Importance of the Issue
It is considered to be a good practice to refer to third-party reputable websites for a comprehensive topic coverage of a specific page, or backlink a source of information. Nevertheless, employment of too many external references might make a search engine treat a page as spam, therefore having a negative impact over the entire website.
John Mueller, a Google employee, tells about outbound references:
“I would watch out for a few types of references, though. In particular, if you’re linking out to a site because of an arrangement like “You link to me, and I’ll link to you.”
Or because it’s an advertisement or if it’s being done in your site’s comments, and you’re not really sure how good those references are.
In all of these situations your pages aren’t linking out because you think the other page is a great match for users.
For these kinds of links we recommend using the rel nofollow link attribute.
In the most common cases however if you’re linking out naturally from your content to other sites that offer additional value and more context then that’s fine and there’s nothing special that you need to do.
How to Check the Issue
With Sitechecker, you can explore external references from your pages. It analyzes your website and identifies pages with a range of issues, including those containing over 100 external links.

Of course, you will receive a list of all external links on each affected page.

How to Fix This Issue
Study all external links at a current page. Leave only those references that can actually help a user or point to the content author. Check if there are any references you haven’t set. The presence of such links, as well as ones to 18+websites, might indicate that your website has been hacked.
Conduct an audit on your website to detect high external linking problem!
Make a full audit to find out and fix your technical SEO in order to improve your SERP results.