There isn't anything you can do because the whole domain is offline meaning that the developers have problems with hosting or it has completely expired. In this case you have to obtain another application. You can install Thunderbird to configure the account and then delete the emails in a batch mode or using Select All. Also, connect to IMAP and change the settings of the email service to allow the full access to IMAP protocol when 3rd party apps are going to be used.
Download Thunderbird: https://www.mozilla.org/en-US/thunderbird/