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 support Gmail, but we'll be adding compatibility with other email providers soon.