When sending marketing messages we connect the system to your email provider. Therefore the limits vary depending on your provider.
If you connect it to a regular provider such as your gmail account, you might only be able to send a few hundred emails per day.
But if connected to a professional level service such as Mailgun, Amazon SES, SendGrid etc. you can send tens of thousands of emails per day - assuming you have that many members of course!
Standard transaction emails such as membership confirmations and reminders are sent through our own email system. Only marketing campaign emails need to be connected to your email provider.