Access webmail (OWA) by going to https://outlook.office365.com. You must use your UPN (full email address unless otherwise specified) as the username along with your network password to login. Using domain\username will not allow you to log in to Exchange Online.
Please note: Accessing any cloud resources (webmail, Sharepoint Online, CRM Online, etc.) outside of the State of Indiana network will require users to be enrolled with multi-factor authentication.