How do I receive a copy of the mail quota emails sent to my customers?

If you'd like to receive a copy (via Bcc) of the emails sent to your customers warning them that they're approaching their mailbox quota, you can opt into these by heading to Reseller Preferences > Reseller Customisations.

Locate the section entitled Customer Email Options and select the checkbox to Bcc your My20i account email address into those emails. Your 20i email address will be added as a Bcc address to emails sent to your StackCP Users about their mailbox usage. It will allow you to monitor and potentially upsell customers a mailbox upgrade.