Attention!
What this code does is: every time the magento-cronjob removes all sent messages from the core_email_queue database table, it also removes all recipients of these messages. So, basically, it does not work for you until this cronjob-task has run at least once.
Solution
I found the answer thanks to another question here: the core_email_queue_recipients table was not emptied by the cronjob. The method Mage_Core_Model_Email_Queue::cleanQueue() calls Mage_Core_Model_Resource_Email_Queue::removeSentMessages(), which is pretty simple:
public function removeSentMessages() {
$this->_getWriteAdapter()->delete($this->getMainTable(), 'processed_at IS NOT NULL');
return $this;
}
Anyway, this method does not remove the old recipients. Thus, as soon as a new message with message_id n is queued, all old recipients with message_id n will also get the new email. The thing I don't understand is: why has the core team not seen this, and why doesn't this lead to more issues?
I wrote a small module to fix this. It uses a class override for Mage_Core_Model_Resource_Email_Queue, so if anybody can suggest a better (event-based?) solution, I would be glad.
app/code/local/Namespace/EmailQueueFix/etc/config.xml
<?xml version="1.0"?>
<config>
<modules>
<Namespace_EmailQueueFix>
<version>1.0</version>
</Namespace_EmailQueueFix>
</modules>
<global>
<models>
<core_resource>
<rewrite>
<email_queue>Namespace_EmailQueueFix_Model_Resource_Email_Queue</email_queue>
</rewrite>
</core_resource>
</models>
</global>
</config>
app/code/local/Namespace/EmailQueueFix/Model/Resource/Email/Queue.php
<?php
class Namespace_EmailQueueFix_Model_Resource_Email_Queue extends Mage_Core_Model_Resource_Email_Queue {
/**
* Remove already sent messages
* ADDED: also remove all recipients of sent messages!
*
* @return Mage_Core_Model_Resource_Email_Queue
*/
public function removeSentMessages() {
$writeAdapter = $this->_getWriteAdapter();
$readAdapter = $this->_getReadAdapter();
$select = $readAdapter->select()->from(array("ceqr" => $this->getTable('core/email_recipients')), array('*'))->joinLeft(array('ceq' => $this->getMainTable()), 'ceqr.message_id = ceq.message_id', array('*'))->where('ceq.processed_at IS NOT NULL OR ceq.message_id IS NULL');
$recipients = $readAdapter->fetchAll($select);
if ( $recipients ) {
foreach ( $recipients as $recipient ) {
$writeAdapter->delete($this->getTable('core/email_recipients'), "recipient_id = " . $recipient['recipient_id']);
}
}
$writeAdapter->delete($this->getMainTable(), 'processed_at IS NOT NULL');
return $this;
}
}
app/etc/modules/Namespace_EmailQueueFix.xml
<?xml version="1.0"?>
<config>
<modules>
<Namespace_EmailQueueFix>
<codePool>local</codePool>
<active>true</active>
</Namespace_EmailQueueFix>
<depends>
<Mage_Core/>
</depends>
</modules>
</config>
