High-Performance Email Delivery System

The Challenge

iknow-uk, a growing accommodation listing platform, needed to scale their email marketing system to handle over a million scheduled emails while maximizing user engagement. The existing standard email system couldn't handle the volume and sent emails at arbitrary times, resulting in poor open and click-through rates.

The Approach

I designed and built a custom email delivery engine from the ground up using PHP and MySQL, with direct SMTP server integration for maximum control and performance.

Key Technical Decisions

• Direct SMTP communication: Bypassed standard mail libraries to implement custom retry logic and dynamic throttling
• Intelligent send-time optimisation: Analysed historical user engagement data to determine the optimal delivery time for each subscriber
• Dynamic backoff: Implemented adaptive rate limiting that responded to SMTP server feedback and temporary issues
• Bounce tracking: Built comprehensive bounce handling and list hygiene automation
• Scalable architecture: Designed for horizontal scaling with queue-based processing

Implementation Highlights

• Custom PHP mailer with direct socket-level SMTP control
• MySQL-backed queue system with priority scheduling
• Per-user engagement tracking and send-time prediction
• Real-time monitoring and alerting dashboard
• Automated retry and error recovery mechanisms

The Outcome

The system transformed iknow-uk's email marketing effectiveness:

• 50,000+ emails per hour sustained throughput with room to scale further
• 100% uptime over 6 years of continuous operation before company acquisition
• Several-fold increase in accommodation enquiries compared to the previous system
• Intelligent delivery: Each subscriber received emails at their personally optimal time based on past behaviour

The system ran flawlessly from deployment through iknow-uk's acquisition by Snaptrip, handling millions of emails and directly contributing to the company's growth and eventual exit.

Technical Stack

• Backend: PHP (custom SMTP implementation)
• Database: MySQL (queue management, user tracking, analytics)
• Infrastructure: Linux servers with Apache
• Monitoring: Custom dashboards and alerting

Lessons Learned

• Measure user behaviour: The send-time optimisation feature delivered outsized impact by using actual engagement data
• Build for reliability: The 6-year zero-downtime run proved the value of thorough error handling and monitoring
• Know when to build: A custom solution was justified here because standard tools couldn't deliver the performance and intelligence required