Simple Email Queue is a WordPress plugin (download) that used to pass restrictions set by your host on number of sent emails in given period.
This free version is only useful for developers. If you want to use it with in full capacity, consider buying premium version.
First, you need to set what is maximum number of emails that can be sent in given period. By default, it sends 10 emails in 6 minutes. This means that if you want to send 15 emails in 6 minutes, only 10 emails are sent while rest 5 emails are put in queue and sent in next hour.
You can change this limits by using filters from you code. If you want user interface in your admin, use premium version.
Filter simple_email_queue_max is used to set maximum number of emails that is sent in period. It expects positive integer to be passed.
To change period length, you can use filter simple_email_queue_interval. It also expects positive integer to be passed but please take care that this number is of seconds, not minutes (for example, if your period is 30 minutes, you would pass 1800).
Different hosts use different limits. Please consult your host’s documentation or support to find this out. For example, SiteGround for its StartUp and GrowBig WordPress hosting plans is having a limit of 400 emails sent per hour (hover over “Email” row texts to find this out).
To use this limits, you need to use function simple_email_queue_add() instead of built-in function wp_mail(). Both accept same parameters. Emails that are sent using wp_mail() function are not sent using queue. If you want that all emails are sent using queue, even those sent using wp_mail() function, use premium version.
Simple Email Queue is released under the terms of GNU General Public License. It is completely free so if you find it useful you can give something back to author. Your help could be done by giving donation, sending pull request on GitHub, sending bugs and suggestions, promoting plugin in blogs, forums, social networks, and other websites.