Your username is the actual name of the mailbox on the Exchange server. This name was set by your account administrator when your mailbox was created. It could be in the format of firstname.lastname, or name3, and is unique to your mailbox. For example, if your name is John Smith, your user name could be john3 or john.smith.
You can find your username and Exchange domain in either HostPilot under MS Exchange Server > Mailboxes > Settings > Advanced tab, or My Services > Advanced.
Your primary email address (in our environment, this is also your User Principal Name or UPN) is associated with your mailbox username. It will also appear in the From field of the messages others receive from your mailbox. You will be able to receive messages using other email addresses, but not send messages from them.
You can find your primary email address either in HostPilot® Control Panel under MS Exchange Server > Mailboxes > Settings > Email Addresses tab, or in My Services > Email Addresses.
In order to log in to your Exchange mailbox, you need to use either your Exchange domain and username, or your primary email address. Read the Knowledge Base article on What is the Exchange Domain, and why is it important? for more information on the domain.