While talking to my maaavelous host
SkyToaster tonight, I found out that there are no throttling settings within MODX. Servers are pretty sticky about mails/hr, and with good reason.
I'm working with a client with over 3700 emails in a list. Currently I use poMMo for their mailings and while I may be happy to stick with that, what I really want is an all in one mailing system through MODX. Which is what I think you're looking for.
Patrick suggested I look into
Mandrill They offer 12K emails per month for free. It's a transactional service by default, yet it looks as if sending mailings/newsletters is also supported.
I've signed up my client email there and am working on a dev site attempting to implement this. So far, I'm not mailing, but since
SkyToaster is such a freakin awesome host, Patrick is going to help me with the settings after he sleeps. I'll report back with results soon. I do recommend checking it out though if you're going to do mailings through MODX.
There's always the integrated packages for Mailchimp of course, but a large list is going to cost your client significant monthly fees with a service like Mailchimp. IMO, if you're promoting MODX mailing lists, there should be no additional costs beyond development.