Risks of mass sending
Front does not have a sending limit, but depending on the type of channel you are sending from, you could run into issues when trying to send a high volume of emails at the same time. For example, there is a rate limit for API-based channels, recipient limit for Gmail channels, etc.
When this happens, Google returns a generic error making it hard for Front to tell you exactly what's happening. So, if you experience issues while mass sending, please look in to your email provider's sending limits. In addition, here are Twilio's rate limits when sending mass messages and Front's API rate limit.