What non-functional requirements do you consider when designing an email sending system?

When designing an email sending system, it is important to consider the following non-functional requirements:

• Reliability: ensure delivery of emails, error handling, and retry mechanisms.
• Scalability: ability to handle a large volume of emails as load increases.
• Performance: minimal email delivery time.
• Security: protection against unauthorized access, spam prevention, using SPF, DKIM, DMARC.
• Monitoring and logging: tracking delivery statuses, errors, analytics.
• Queue management: to prevent overload of SMTP servers and control sending speed.
• Compatibility: support for various email formats (HTML, text), encodings.
• Fault tolerance: component redundancy so the system continues to operate during failures.

Example: if the system needs to send notifications to millions of users, a distributed architecture with message queues and load balancing should be considered.