Stabilizing email delivery by addressing reputation, infrastructure, and sending behavior
This project focused on improving email deliverability for large outbound sends that were being blocked or throttled. The goal was to establish a reliable sending approach that could reach the full audience without triggering reputation issues.
Identify the Constraints
- Reviewed bounce codes and delivery failures (S3150, 5.7.28)
- Identified IP reputation and rate limiting as primary constraints
- Noted conflicting DNS and sending infrastructure across providers
- Observed that large batch sends were consistently triggering blocks
Adjust the Strategy
- Replaced single large sends with segmented delivery batches
- Established a controlled send rate based on observed throughput
- Timed sends to allow reputation recovery between segments
- Avoided reliance on default tool behavior in favor of deliberate control
Validate & Refine
- Monitored delivery rates and bounce patterns across segments
- Compared segmented results to prior full-list sends
- Adjusted batch size and timing based on real performance
- Established a repeatable approach for future sends
Refine
Challenge
Email sends were failing due to reputation-based blocking and rate limiting, particularly from Microsoft servers. Large batch sends resulted in high volumes of soft bounces, making it difficult to reliably reach the full audience.
Insight
The issue was not the email content or the tool itself. It was the interaction between:
- IP reputation
- sending volume
- and delivery timing
Sending the full list at once consistently triggered throttling, regardless of message quality.
Solution
Using MassMailer, the sending approach was restructured around controlled distribution rather than single large sends.
- Broke large sends into smaller segments
- Aligned sending volume with observed delivery limits
- Introduced pacing between sends to reduce throttling
- Identified infrastructure conflicts as a contributing factor
The focus shifted from “sending all at once” to delivering reliably over time.
Result
With a controlled sending strategy in place, delivery became predictable and scalable:
- Soft bounces were significantly reduced compared to full-list sends
- Delivery rate stabilized at approximately 500 emails per 10 minutes
- Large lists were successfully delivered over time instead of failing in bulk
- Sending performance could be measured and adjusted as needed
Most importantly, email delivery became something that could be managed intentionally rather than left to chance.