You can be logged in to multiple accounts at the same time when using the Fastmail web interface, desktop app, or mobile app.
Select user
If you go to Fastmail's login screen while already logged into an account, you will be brought to our Select user page. This page shows any accounts you are currently logged in to. Select one to go to the account's Inbox.
To go to another account that is not currently logged in, click Log in as another user at the bottom of the list.
Switching between accounts
You can also switch between logged in accounts while accessing the Fastmail interface. The exact steps for this will look different depending on whether you are using the Fastmail web interface or the desktop/mobile app. For more information, please see our Switching users help page.