(Please don't read this and think that my tone is shouting at you.... I am just saying what happens when these messages arrive here. OK? Also, I am not an SEO, I am a publisher. So that puts a little different perspective on my thinking. )
When an SEO or a PR company contacts me about "guest posting" on my blog I report that message as spam and block their email address or entire domain from my inbox.
I don't want posts that are done to connive links. I don't need content that badly and if the post is being done to gain a link I bet that they didn't put their best work into it. What else are they doing for links? Will I be linking to a bad neighborhood? Forget it.
I do publish a lot of content contributed by others. But every one of those authors contacted me personally. Sometimes I accept their first article and pay them for future articles.
When I publish that content from a guest author there is an area on the page with a short author bio and a link to his/her blog or website. I don't mind linking to his/her site. Maybe my visitors want to read more from that author. Good!
However, when an author contacts me personally if he/she starts talking about getting anchor text links in the content my interest in that content falls through the floor. Now I know why he wants to have an article on my site. He simply wants the links. FT! I am not running a website to be JoeSchmoe's personal advertising portal or his personal linkfarm - those articles submitted for links are usually crap content. I don't want them. Sorry, we can't use the article.. thanks for offering it to us.