What are the limits of sending mails via SMTP?

What are the limits of sending mails via SMTP?

Hello,

Basically speaking I'm using zoho.com free account via SMTP at this moment for sending my application and board registration mails to new users. I'm wondering what are the exact limits and what happens to mails once limit is exceeded?
Am I able to see how much mails are still allowed to send?

Do Premium account have higher limits, etc? I would like to know all the details.

Thank you.