Limit Outgoing Messages (QMail)

We've got a dedicated server at our company that hosts several sites and email accounts. Today I noticed that in the mail queue (from Plesk) we have like 5 or 6 messages from the same customer with around 400 destination addresses for EACH one. This would be like 2000-3000 emails to be sent. It isn't spam as it's some kind of newsletter.

I don't really know how the QMail server handles this, but it's been 5 hours since some of those messages entered the queue, and they are still there, so it seems that is having some difficulty.

I don't pretend to limit the amount of emails an user can send per minute or per hour, but I would like to know if there is any way of managing the queue like, lets say, send 50 message per minute. As far as I know, the mail queue right now (by default) starts sending the messages as they come, which means it could send 1000 in a few seconds if it can handle it.

I don't even know if this would be better or worse, meaning that maybe messages could get queued when the server could handle them, so some customers would see that emails are not working instantly as they do now. It also would be good if this tool (if exists) could report the current status of the queue, saturation, etc...

We're a bit worried because a couple of days ago we had some kind of attack and our server started sending hundreds of emails with fake sender and the CPU went overloaded and the mail queue was too big.