Mailer is a plugin that queues WordPress emails (send by WordPress or by plugins via wp_mail() function) and “slowly” send the to stay under the provider limit. Hitting a provider mail per hour limit means to loose those emails.
Version 1.4.0 is totally new an solves a number of problem due to the change of the mailing library included in WordPress.
For user who have an older version and DO NOT see the update notice, just delete the plugin (nothing will be lost) and install it again.
Mailer can be configured to use an external SMTP as well.