What is it?
In cases where you are using more then one E-mail Accounts , this field is used to identify which E-mail Account will be identified to the Recipient by appearing in the From field. This will also be the E-mail Account to which the recipient will reply if using the Reply or Reply All feature.
What happens if clicked?
A list of available E-mail Accounts will open. You can select the desired E-mail Account from the drop down menu.