That's what I'm leaning towards, too. The best user experience is not having to watch your back in terms of how many threads you've used this month, so I think it's better to do it by users.

I think we're going to have to limit free plans, though.

Don't limit free plans by threads. It's much better to limit them by, for example, not allowing customization, showing a big "powered by you" button, adding a footnote (powered by) in each email, stuff like that. Make me pay 9.99$/m if I want to look "professional" and not using a free service. Make me pay $29,99 for unlimited support accounts or something.

Not a bad idea — one of the things we wanted to limit to higher accounts was whitelabeling. Re: no free plan: I don't know if this would work - won't people want to try it longer, and wouldn't a free plan work towards our favor in word-of-mouth?

Or even better, don't have free plans, have free trials. 30 day free trial, then 9.99$/m The non-payers will leave after 30 days, which is fine.

Switching support software isn't gratis, 30 days free trial may make people look at it half-heartedly, but not really try to integrate it.