suppose the following are the use cases -

• send and receive an email
• add an attachment

The following is the usage -

• 100M users and 10M active users
• 30M emails per day
• 1 in 5 has an attachment to it - attachment max size is 2KB
• email content avg size is 500 chars

Design an Email system with the above requirement
Please share your answers as I am not sure what would be the best possible answer.