Design a Scalable Email Service

Functional Requirements:

• Users can send and receive emails.
• Support for basic email folders (Inbox, Sent, Trash, etc.).
• Ability to search emails by subject, sender, or body.
• Mark emails as read/unread.
• Support for email attachments.
• Basic spam filtering.

Non-Functional Requirements:

• High availability: Email service should be accessible at all times.
• Scalability: System should handle growth in users and email volume.
• Reliability: No loss of email data.
• Low latency for common operations (fetching, sending emails).
• Security: Emails and attachments should be protected from unauthorized access.
• Durability: Emails and attachments must be stored persistently.