Sending limits can be configured from here.
The maximum sending limit is 400 emails/day. Prospect.io enforces this limit in order to preserve your email account. Above 400 per day, the emails you send might be considered SPAM by the recipient's server or, your email service provider may prevent you from sending more emails.
In order to send more emails, you should dispatch them on several sending addresses.
Little note about Gmail:
Gmail allows between 500 and 2000 emails per day. However, in order to ensure our customers never reach the lower limit (500), we limit the emails sent by Prospect.io to 400 per day.
For more informations about Google limit, here is the article:https://support.google.com/a/answer/166852?hl=en)