This MOD does not install cleanly. This is because install.xml is looking for spaces instead of tabs, as well as inexistent spaces at the end of the lines.
The fixed install.xml (for the current code of course) could be something like this
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE modification SYSTEM "http://www.simplemachines.org/xml/modification">
<modification xmlns:smf="http://www.simplemachines.org/" xmlns="http://www.simplemachines.org/xml/modification">
<id>von-COLLINZO:DeleteAllMailQueue</id>
<version>1.0</version>
<file name="$sourcedir/ManageMail.php">
<operation>
<search position="after"><![CDATA[ 'clear' => 'ClearMailQueue',]]></search>
<add><![CDATA[
//Begin: von COLLINZO: Delete all Mail Queue button
'clear2' => 'ClearMailQueue2',
//End: von COLLINZO: Delete all Mail Queue button
]]></add>
</operation>
<operation>
<search position="replace"><![CDATA[ 'value' => '[<a href="' . $scripturl . '?action=admin;area=mailqueue;sa=clear;' . $context['session_var'] . '=' . $context['session_id'] . '" onclick="return confirm(\'' . $txt['mailqueue_clear_list_warning'] . '\');">' . $txt['mailqueue_clear_list'] . '</a>] <input type="submit" name="delete_redirects" value="' . $txt['delete'] . '" onclick="return confirm(\'' . $txt['quickmod_confirm'] . '\');" class="button_submit" />',]]></search>
<add><![CDATA[ //Begin: von COLLINZO: Delete all Mail Queue button
'value' => '[<a href="' . $scripturl . '?action=admin;area=mailqueue;sa=clear;' . $context['session_var'] . '=' . $context['session_id'] . '" onclick="return confirm(\'' . $txt['mailqueue_clear_list_warning'] . '\');">' . $txt['mailqueue_clear_list'] . '</a>] [<a href="' . $scripturl . '?action=admin;area=mailqueue;sa=clear2;' . $context['session_var'] . '=' . $context['session_id'] . '" onclick="return confirm(\'' . $txt['quickmod_confirm'] . '\');">' . $txt['delete'] . ' ' . $txt['all'] . '</a>] <input type="submit" name="delete_redirects" value="' . $txt['delete'] . '" onclick="return confirm(\'' . $txt['quickmod_confirm'] . '\');" class="button_submit" />',
//End: von COLLINZO: Delete all Mail Queue button ]]></add>
</operation>
<operation>
<search position="end" />
<add>< );
redirectexit('action=admin;area=mailqueue');
}
//End: von COLLINZO: Delete all Mail Queue button ]]></add>
</operation>
</file>
</modification>