Mail Queue

You can enable the Queue, control how many emails and how often they should be sent.

You can enable the Alerting feature and control at which point exactly you want to be alerted.
How does this plugin work?
When enabled, the plugin intercepts wp_mail(). Instead of sending emails immediately, they are stored in the database and released gradually via WP-Cron according to your configured interval.
Does this plugin change the way HOW the emails are sent?
No. This plugin does not change HOW the emails are sent. For example: If you use SMTP for sending, or a third-party-service like Mailgun, everything will still work.

This plugin changes WHEN the emails are sent. By the email Queue it gives you control about how many emails should be sent in which interval.
Does this plugin work, if I have a caching Plugin installed?
If you're using a caching plugin like W3 Total Cache, WP Rocket or any other caching solution which generates static HTML files and serves them to visitors, you'll have to make sure you're calling the wp-cron file manually every couple of minutes.

Otherwise your normal WP Cron wouldn't be called as often as it should be and scheduled messages would be sent with big delays.
What about Proxy-Caching, e.g. NGINX?
Same situation here. Please make sure you're calling the WordPress Cron by an external service or your webhoster every couple of minutes.
My form builder supports attachments. What about them?
You are covered. All attachments are stored temporarily in the queue until they are sent along with their corresponding emails.
What are Queue alerts?
This is a simple and effective way to improve the security of your WordPress installation.

Imagine: In case your website is sending spam through wp_mail(), the email Queue would fill up very quickly preventing your website from sending so many spam emails at once. This gives you time and avoids a lot of trouble.

Queue Alerts warn you, if the Queue is longer than usal. You decide at which point you want to be alerted. So you get the chance to have a look if there might be something wrong on the website.
Can I add emails with a high priority to the queue?
Yes, you can add the custom X-Mail-Queue-Prio header set to High to your email. High priority emails will be sent through the standard Mail Queue sending cycle but before all normal emails lacking a priority header in the queue.

Example 1 (add priority to Woocommerce emails):

add_filter('woocommerce_mail_callback_params',function ( $array ) {
$prio_header = 'X-Mail-Queue-Prio: High';
if (is_array($array[3])) {
$array[3][] = $prio_header;
} else {
$array[3] .= $array[3] ? "\r\n" : '';
$array[3] .= $prio_header;
}
return $array;
},10,1);

Example 2 (add priority to Contact Form 7 form emails):

X-Mail-Queue-Prio: High

Example 3 (add priority to WordPress reset password emails):

add_filter('retrieve_password_notification_email', function ($defaults, $key, $user_login, $user_data) {
$prio_header = 'X-Mail-Queue-Prio: High';
if (is_array($defaults['headers'])) {
$defaults['headers'][] = $prio_header;
} else {
$defaults['headers'] .= $defaults['headers'] ? "\r\n" : '';
$defaults['headers'] .= $prio_header;
}
return $defaults;
}, 10, 4);

For this you can add the custom X-Mail-Queue-Prio header set to Instant to your email. These emails are sent instantly circumventing the mail queue. They still appear in the Mail Queue log flagged as instant.

Mind that this is a potential security risk and should be considered carefully. Please use only as an exception.

Example 1 (instantly send Woocommerce emails):

add_filter('woocommerce_mail_callback_params',function ( $array ) {
$prio_header = 'X-Mail-Queue-Prio: Instant';
if (is_array($array[3])) {
$array[3][] = $prio_header;
} else {
$array[3] .= $array[3] ? "\r\n" : '';
$array[3] .= $prio_header;
}
return $array;
},10,1);

Example 2 (instantly send Contact Form 7 form emails):

X-Mail-Queue-Prio: Instant

Example 3 (instantly send WordPress reset password emails):

add_filter('retrieve_password_notification_email', function ($defaults, $key, $user_login, $user_data) {
$prio_header = 'X-Mail-Queue-Prio: Instant';
if (is_array($defaults['headers'])) {
$defaults['headers'][] = $prio_header;
} else {
$defaults['headers'] .= $defaults['headers'] ? "\r\n" : '';
$defaults['headers'] .= $prio_header;
}
return $defaults;
}, 10, 4);

Exceptions:

If for some reason the email cannot be entered into the database, wp_mail() will return false.

However if you send an email using the instant header option the email will be considered important.
In this case the email will be sent right away, even if there is an error creating a log for it in the queue.
I have a MultiSite. Can I use Mail Queue?
Yes, but with limitations.

Do not activate the Mail Queue for the whole network. Instead, please activate it for each site separately. Then it will work smoothly. In a future release we'll add full MultiSite support.
What is Mail Queues favorite song?
[youtube http://www.youtube.com/watch?v=425GpjTSlS4]