Help solving WordPress MyMail Email Campaign Cron Problem

General information for the website: Restaurant Newsletter
Num. of web pages/modules: 1
Description of every page/module: Newsletter Settings Page
Description of requirements/features: Hello,

I have setup an email newsletter for a restaurant using the WordPress email newsletter plugin MyMail. The newsletter is scheduled to go out to 300,000 subscribers but it is sending very slowly after 4 days it has only gone to 1,532 subscribers.

I am getting an error message:
It seems your last cronjob hasn't been finished! Increase the 'max_execution_time', add define("WP_MEMORY_LIMIT", "256M"); to your wp-config.php or reduce the Number of mails sentin the settings

This setting has been added to the wp config file but I still get the same error message.

I am looking for assistance by somebody used to solving these issues. Somewhere in the process the sending rate is being severely limited and I would like to resolve the issue so it sends more promptly.

I have also added a custom setting to the ht access file using the 'WP htaccess Control' plugin:

php_value max_execution_time 256M

Unfortunately neither fix seems to have worked.

If you can help please reply with a rate to fix the issue and your requirements?

Thanks,

David
Extra services needed: Customization
Specific technologies required: PHP, CRON
Extra notes: