📬 Deliverability Engineering

A/B Testing Subject Lines: Multi-Armed Bandit Algorithms for Outreach Optimization

👤 Author: Chief Deliverability Architect & Lead Systems Engineer📅 Technical Review: September 2026⚡ DMARC & RFC 8058 Compliant

Traditional A/B testing splits traffic 50/50 until statistical significance is achieved, wasting valuable leads on underperforming copy. Dynamic outreach engines employ Multi-Armed Bandit (MAB) algorithms that automatically route traffic toward the highest-converting subject lines in real-time.

1. Thompson Sampling in Email Subject Line Optimization

Thompson sampling models conversion rates as Beta distributions $\text{Beta}(\alpha, \beta)$, where $\alpha$ represents positive replies and $\beta$ represents un-replied sends. The algorithm draws random samples from each distribution and selects the subject line with the highest simulated yield, continuously adapting to live recipient feedback.

Robert Baindourov

Written by Robert Baindourov & ContactCampaigns Deliverability Council

Senior outreach systems architect and email deliverability consultant specializing in Postfix/Haraka MTA optimization, SPF/DKIM/DMARC cryptographic alignment, and CAN-SPAM / GDPR international compliance.