Mail Queue

You can choose whether the Mail Queue should be Enabled, Paused, or Disabled, and you can control how many emails are sent and how often they should be sent.

You can also enable alerts, define the queue size threshold for alerts, and optionally enable automatic pausing when an alert is triggered.
How does this plugin work?
When the Mail Queue is 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.

The plugin offers three states:

Enabled: emails are added to the queue and sent gradually by WP-Cron.
Paused: emails are still added to the queue, but no queued emails are sent until you enable the queue again.
Disabled: the plugin does not intercept wp_mail() and has no effect on outgoing emails.
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?
Queue alerts are a simple and effective way to improve the security of your WordPress installation.

Imagine your website starts sending spam through wp_mail(). The Mail Queue would fill up quickly instead of sending everything at once. This gives you time to react, avoid a lot of trouble and can reduce the damage significantly.

Queue alerts warn you when the queue grows longer than usual. You configure in the settings at which threshold you want to be alerted. This gives you the chance to review the queue and investigate whether something unusual is happening on the website.

Optionally, you can enable automatic pausing when an alert is triggered. In that case the queue stops sending emails until you change the status back to Enabled.

Please note: This plugin sends at most one alert every six hours while the queue remains above the configured threshold.
Can I pause the queue without disabling the plugin?
Yes. The Mail Queue can be set to Paused.

When paused, the plugin still intercepts wp_mail() and stores outgoing emails in the queue, but no queued emails are sent until you change the status back to Enabled.

This is useful if you want to temporarily stop outgoing delivery without disabling the plugin completely.
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 are still processed through the normal Mail Queue cycle, but they are sent before normal queued emails.

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

For this you can add the custom X-Mail-Queue-Prio header set to Instant to your email. These emails are sent immediately and bypass the queue. They still appear in the Mail Queue log so that their delivery remains visible.

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

Can I send queued emails immediately from the backend?
Yes. In the Queue tab you can select one or more queued emails and use the Send now bulk action.

This sends the selected queued emails immediately without waiting for the next WP-Cron cycle.

The action is only available for queued emails that are still waiting in the queue.
What are queue events in the log?
In addition to email entries the Mail Queue log also shows queue events.

These entries document important queue state changes, for example when the queue was enabled, paused, disabled, and auto-paused automatically after an alert.

They are not outgoing emails. They are informational log entries to help you understand what happened and when.
Can I still use the wp_mail() function as usual?
Yes, the wp_mail() function works as expected.

Exceptions:

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

If you explicitly send an email using the Instant priority header, the email is sent immediately instead of being queued, 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 Queue's favorite song?
[youtube http://www.youtube.com/watch?v=425GpjTSlS4]