Case Study: Email Delivery Issues on a High-Volume Mail Server

17.01.2026

Background

A high-volume mail server was experiencing delayed and rejected outbound emails. While the server resources were stable, email delivery rates were inconsistent, causing critical communication delays.

Initial Findings

  • Mail queue size growing intermittently

  • No CPU or memory bottleneck

  • Outgoing emails delayed or partially rejected

Investigation Steps

  • Mail queue and log analysis

  • Review of sending IP reputation

  • SPF, DKIM, and DMARC validation

  • Rate limiting and policy inspection

  • Blacklist and feedback loop checks

Root Cause

The issue was not server performance-related but caused by reputation degradation and policy misalignment on high-volume outbound traffic.

Resolution

  • Queue optimization

  • Policy and rate adjustments

  • Authentication alignment (SPF/DKIM/DMARC)

  • Reputation cleanup and monitoring

Key Takeaways

  • Email delivery issues are often reputation-based, not hardware-based

  • High-volume mail systems require proactive monitoring

  • Small misconfigurations can have large operational impact

Share