This area is used to create e-mail accounts to use for outgoing mail. It is not for importing addresses that you will mail to! Be extremely careful with this interface. It can create thousands of e-mail accounts on your domain within a few minutes. If you make a mistake and add a lot of unwanted accounts to an add on domain then you can remove them by going to Add On Domains and removing the domain then adding it back afterwards. If you add unwanted e-mail addresses to your principal domain, you will have to delete them one at a time.
To import addresses, browse to a csv or xls file on your computer, select the delimiter (if you are not sure you should be able to tell by examining the file in a text editor, normally it is either comma or tab), check off whether to Treat first row as column headers and click next.
You will see dropdown lists at the top of each column labelled Ignore, Email, Password, Domain, and Quota. Select the appropriate one for the columns you need and leave the others set to ignore. Adjust the dropdown box (if present) for the correct domain. Then click next. You will be taken to the next page where you can watch as the addresses are created. Now go back to Email Accounts to see the new accounts and add or edit passwords if needed.
Besides creating e-mail addresses this way, you may also import a list of auto responders.