Outpost is a communication management tool. It operates as a layer on top of your email account. So Outpost itself is not an email provider - instead, it adds tools and visibility that help your team coordinate efforts in your existing email accounts.
You will still need a separate email account for each mailbox you'd like to create in your Outpost account. You can create a new email account (or several) to use with Outpost, or import an email account you've had for some time.
Currently, Outpost supports Gmail and Office 365, but we'll be adding compatibility with other email providers soon.
This video shows you how Outpost works: