Hi Anson,
Didn’t end up using Firemail for this particular project as the client didn’t want to be a ‘guinea-pig’, so they went with Mailchimp. Still waiting on the Mailchimp EE module but I’ve heard its coming soon.
Have been talking to a server technician about it though, and as long as you have a decent server (an EE site deserves a VPS) there shouldn’t be any problem with sending mail to 10000+. Should be fine as long as you’re not deliberately sending spam (which could result in your entire IP being blacklisted). And even then, you can get back off the black-lists relatively easily (assuming you’re legit). So I’m planning on using Firemail for the next big project, with a list of 7500 - you just can’t beat the benefit of being able to do everything from within the EE control panel.
So I reckon Firemail is worth a shot, and if theres any issues you can always go back to Mailchimp or campaignmonitor modules, which will both be released soon. Depends if the client is someone you have a good relationship, who understands the minor risk vs possible benefit. If its an extremely formal, critical project, probably best to use one of the big mail companies