You can switch to this account within your Webmail login. (Note: this is not the same for mobile)
Log into Webmail, click on the Portrait icon found in the top right corner.
Click on "Open another mailbox...", this will open a dialog box, type in the email address or the Display Name of the account and click the "Open" button. This will reload the page in that email account. You can use it to create Out-of-office messages and make appointments in the calendar.