I don't exactly know what the upper limit is, but a month or two ago one of our customers used the system to send a distribution to nearly 500,000 subscribers without a hitch. It took a while for everything to go out, but I'd be surprised if you have any problems with a list of 10,000. :>
While I'm sure you don't intend to spam, you may still want to check out our anti-spam policy (since there are a few common definitions of spam floating around):
Also, we've recently begun asking those who send large distributions to keep 'confirmed opt-in' turned on with their lists. This helps avoid accidental subscriptions, and makes sure that only people who want your newsletter receive it. This is the default option when you set up the list.
- Jeff @ DreamHost
- DH Discussion Forum Admin