With users having access to multiple accounts and multiple computers, there will be a time where Outlook prompts for a Domain Name when you need to use it. This is normal security feature and can often be solved by requesting the network account be unlocked and reset the password if it is currently not known.
If you see this happening frequently these are the possible cause.
You have a mobile device that is checking for email with the incorrect password. When this is attempted (usually 10 times), it will lock the account for your protection, you will need to disable sync and then update the password in the mobile device and then enable sync so you can be alerted to future messages.
Someone knows your username (and possible old password) and is attempting to get into your account. user names can be guessed as it tends to be the first part of the email address. You IT department will decide if they need to change the username while keeping the email address the same so you will not have to update business cards, contacts on the website and lose incomin mail.