I am reaching out to check if you're accepting content from guest contributions on the topics.

I’m writing to you because I have some really interesting content ideas that I strongly feel will be perfect for your website as well as the kind of audience you are targeting. 

I noticed that you have a few opportunities for guest posting. I'm interested in contributing content to your blog.

We're trying to establish ourselves as helpful, trustworthy and we decided that the best way to do that is through quality content, we have already been featured in respectable publications.