"You may see this message if you send an email to more than 500 recipients in a single email, or send more than 500 emails in a day.
When you get this error, you should be able to send mails again within 1 to 24 hours."
If you need to send an email to more than 500 users, you may need to break the emails into waves of 500 at a time. Alternatively, you may also set up your own SMTP server, and utilize our option for "Custom SMTP." in the Email Users tool.