Outreach currently supports Gmail, Exchange and Outlook as methods to contact prospects or contacts. Each platform has its own set of sending limits, so it is important to understand your organization's limit if a user chooses to send emails in large volumes. See Email Limits & Safeguards.
Google Apps' email limit is 2000 per account, per day (500 for trial accounts). See Google Apps email sending limits.
Microsoft Exchange Server (2010, 2013)
Limits are set by the organization's IT department as the server is hosted by the organization.