The quota is on the ongoing mail, not the storage. Many ISP are using this method to limit spams or virus propagation from their clients. I think google does it

The quota is on the ongoing mail, not the storage. Many ISP are using this method to limit spams or virus propagation from their clients. I think google does it too. But this is not original. On the net I found Freedom, initially published 23 nov 1999 which includes outgoing mail quotas (http://www.homeport.org/~adam/zeroknowledgewhitepapers/arch-notech.pdf http://www.homeport.org/~adam/zeroknowledgewhitepapers/arch-...). For the initial date, check at the bottom.