It's easy to switch between different Mixmax accounts.

In the Mixmax web app, click your avatar photo in the upper right corner, and then select one of the multiple Mixmax accounts you're currently logged into:

To write a new message, click the Compose button in the top left corner of the Mixmax dashboard:

Clicking the Compose button launches the Gmail compose window in a separate popup:

If you want to use multiple email aliases for a single Gmail account — rather than switching between accounts — here's how to add email addresses and aliases to Mixmax.