Emails can be sent out to your StackCP Users warning them when they get close to hitting their mailbox quota. You can CC in a sales or business email address so that if a customer wishes to upgrade it's easy for them to know who to contact. You can set this up by heading to Reseller Preferences > Reseller Customisation. Locate the section entitled Customer Email Options and add the address you'd like to appear to be CC'd into the Mail Quota CC Email Address field, this will usually be something like a sales@ mailbox. Once saved, mail usage emails will contain the CC'd in email address for your customers convenience.