In order to keep things running butter smooth we have to respect the daily sending quota at all times. We've designed a system that detects if you send a campaign with more recipients than what your daily quota can accept.
Don't worry, if this happens, MoonMail queues the pending emails and they're sent after your daily pending quota is up, that is, aka the next day. So, if you see a campaign that's being sent, it may not be an error but just a sign that your daily quota was exceeded. (congratulations btw, means you're doing good business!)
No need to panic :) If you see this happen often drop our support a quick message and we'll allocate a bigger sending quota for you. We totally understand the growth phase and we have plans that can accommodate all your sending needs.