Emails not being sent out since version 2.1.1

I am using Wordpress 3.5.1 and buddypress on an Ubuntu server. The subscribe by email plugin is broke. I had been running the latest version 2.4.? and started getting reports of emails not being sent. I downgraded the plugin to version 2.1.1 and emails started going out again.

Are the newer versions only compatible with WP 3.6 and not 3.5.1? We cannot go to 3.6 until we finish migrating blogs because of a glitch with the wordpress import plugin and anything newer than WP 3.5.2 - it fails to import media.

I have seen other threads and reports of emails not going out. WP System Emails go out. Sending a preview email worked as well. It was just failing to send out to the subscribers.