It refers to something that was once very respectable have a guest article on your blog, as well as get a coveted and respected author to write the introduction to your book. However this is no longer so and will no longer be whether anymore.
It refers also to a very clear example of spam you have received from a vendor working a content marketing agency offers high-end services as a creator of content on your blog (we must have courage to do this) and roughly explains what your way of working.
It is dedicated to the professional search sites to promote their clients and when he found the site of Matt Cutts he was so impressed with the amount of followers you have that gives you the possibility to publish guest posts on your blog, of course 100% original, well-written and relevant to your audience and certainly unique for him. All this in exchange for a dofollow (or two) link in the body of the article and again assured that the quality and content will be relevant to your readers.
According to Matt Cutts if you look at the text highlighted in bold ignoring what others before us someone who is offering us money to get links that pass PageRank. This clearly violates the Google quality guidelines.
That is why now we can not have nice things in the SEO community has fallen into a downward spiral where people post touts outsourcing of guests and writing articles on how to automate guest blogs.
Overall, Matt Cutts recommends the publication of an article by guest unless it is willing to respond personally or by someone knows you well and certainly would not trust such actions as the basis for a strategy of linkbuilding.
This does not mean you can not post as a guest in a blog or you’re the guest in another blog. There is the guest bloggin what has died (or is to kill it), but the guest spam.
It means that the content must be of high quality, relevant and useful to the reader and certainly not be clearly optimized for search engines with SEO purposes.
¿Would reject your guest write an article on a site of much newspaper or magazine type powerful importance only because the links were nofollow? Not you? This will bring relevant traffic to your blog but do not pass PageRank, you would be more visible because it would give to know who are not regular readers to your website.
It also messes with blogs that are made by several authors, blogs can be a lot of quality and certainly very useful for WWW.
Ends clear that a lot of low quality sites or spam blogs have clung to guests as link building strategy and they see it’s a clear attempt to spam to blog quality guest. Ultimately, the goal is not that links “look natural” but the links “are natural”.