Click on Products/Services or follow this link: https://instantbilling.co.za/pay/clientarea.php?action=productdetails&id=133&modop=custom&a=management&page=emails
Login to your account with your email account or username and password.

Click on the email icon
Click on quota of the email account you want to change and change it.

https://instantbilling.co.za/pay/knowledgebase.php?action=displayarticle&catid=10006&id=10004

Make sure that in your email program that you use, under advance settings that you have to have the tick disabled and do not store emails on the server as you will get billed for that bandwidth usage.
In Microsoft Outlook:
Select Tools > Account Settings > double click on the email account in question > Click on more settings > Click on the advanced tab > de-select the option "Leave a copy of messages on the server"
If selected make sure that you specify the time to be not longer than one week.