This post is dedicated to collect comments about Newsletter 2.5+ version. Newsletter Pro 2.5+ has it’s own page here.
Warning: if you update manually, you must DEACTIVATE and then REACTIVATE the plugin. Upgrading form version 1.5.9 require reconfiguration on main panel and on user profile profile panel.
Newsletter 2.5 uses an automated sending engine that works only if the WordPress cron system is working properly. On mail list page, there is a countdown which indicates how many seconds before the next sending engine run. If it is negative (always) your cron system is not working because disabled or because your site has no enough traffic.
On version 2.5.1 there will be a documentation update about it and how to ask the provider to setup a cron for you.