Forget cookie-cutter mass emails (that look like just that) to outreach to the sites. Almost no one will reply, let alone publish the post. Instead of the buckshot approach, be a sharpshooter---study the site's social media and engagement elements. Woo the webmaster or editor with SM messages in conversation for a while, keep things very positive and relevant to the community niche.
Eventually ask, in a natural exchange about contributing to the site, perhaps by first referencing other published content you've written. Play the numbers---for 10-15 guest posts you want, you may need to cultivate relationships with 45-50 different webmasters in a SM campaign (4 supportive tweets to some today, 5 IMs to others tomorrow, etc) until you're ready to try to harvest from this farm. Hope this helps.