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I got the same issue. After the 8th of May, my blog traffic drop nearly 20%. I haven't practiced any black hat seo method. I saw that few other webmasters also reporting the same issue.

My most recent client for unnatural links penalty removal is one that was penalized because of reciprocal linking, mostly on links and resource pages. Doing a couple of link exchanges via a links page is not going to get you a penalty.  But, the Webmaster Guidelines say that you can be penalized for doing excessive link exchange "You link to me and I'll link to you".  They don't say what is considered excessive though. Does this mean you have to get rid of your links page completely?  Not necessarily.  But you have to be careful.  Make sure your links are all relevant.  For example, if a real estate site has a resources page that includes links to flower shops, pet shops and casinos, then this is a red flag for a page that is existing just for the purpose of linking out.  Similarly, if you are getting links back from sites that are totally not related to you then this can be a red flag. It is ok to have a resource page (or to be mentioned on one) when it makes sense and when there aren't LOADS of links on the page. A healthy resource page will not be completely made up of reciprocal links but will also include links to government sites and authority sites in your niche. A better option, though, when trying to exchange links is to exchange guest posts.  Be careful to provide and publish only good quality stuff though....something that your readers will appreciate.  If you have your own blog and provide content for your readers then, when you want to exchange links, ask a partner to write an article for your blog and in return you can do the same.  Be careful not to produce patterns.  For example, don't end every single guest post with the same author bio and anchor texted keywords.  If possible, try to get a link back to your site within the body of the post and not in an author bio.  And again, be careful with your anchor text.

Sounds like a hard to do list but will give it a go as google are putting me out of business at present. Thanks.