If you do not use a local mail client such as MS Outlook, you will need to manually login in to WebMail to manage your POP3 Mail Account.
Here are some instructions on clearing out POP3 Mailbox via Webmail, in this example we are using the very popular Horde Webmail Client.
Clearing our mail is a 2 step process.
1. select mail to delete and click on delete, you will see that then the mail subject line has a line through it

2. you then need to select the sub menu 'purge deleted' from the 'other' top level menu

3. Done
