Outreach currently supports Gmail, Exchange and Outlook mailboxes. Each has its own set of sending limits, so it is important to understand your organization's limit if you choose to send emails in large volumes. See Email Limits & Safeguards.
Google Apps' email limit is 2000 per account per day. If your Gmail account is new or flagged, you are limited to 500 emails per account per day. 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.
300 emails per day, 30 per minute, and up to 100 recipients per email. These limitations are stricter for new or flagged accounts. See Limits for Outlook.com messages.
10,000 per day, 30 per minute and 500 recipients per email. See Recipient and Sender Limits.