5
A customer of mine send thousands in bunches of 100. That is, he clicks on "send", the system send 100, then a window loads with a list of the users mailed and a button saying: click "Next" to send next 100.
It works fine.
The problem is how to test he system without bothering your registered users. You should locate the mail function in the code, comment the line that actually sends the mail and activate the debug console.
Look for this in file "modules/system/admin/mailusers.php"
$xoopsMailer->send(true);
This is the line you should comment, so you DON'T SEND MAILS, but all other processes are done.
Tell us what happens.