Why won't emails send out with a new post? It stopped working.

For 2 years our blog posts were emailed to our subscriber list from the plugin; it worked great. BUT, in the last few months the emails for new blog posts will not send out. We slowed down the batch processing to 30 and it worked a few days then stopped. I just slowed it down to 10 emails every 20 minutes. We are using the most current version of the plugin.

Is this a known issue, or is their resolution for this?