On order to protect email servers from a possible denial of service attack from the blog by email feature of WordPress I have committed a change today to ensure that WordPress will not contact the mail server more that once every 5 minutes however often wp-mail.php is accessed.
The 5 minute interval, is itself, configurable using a define in your
wp-config.php file it you wish to set a longer or shorter interval.
You would configure this by adding the following to the
//Set the minimum interval for wp-mail blog-by-email POP3 connections to 10 minutes define('WP_MAIL_INTERVAL',600);
This feature will be available in WordPress v2.9