Changing the priority of a message on Webmail is a good way to let the recipient know how important it is for them to reply in a timely fashion.
To change the priority of a message:
Select a message (this could be a message you have received, or you could create a new message by pressing the Write icon in your inbox toolbar.)
If this is a message you received, click on the Reply or Forward option in your toolbar.
Once the message is open, click on Options.
Under the Priority option, click on the down arrow to change the priority of the message to Normal, Urgent, or Low.