Definition
Email list decay rate measures how quickly a list loses engagement quality when not actively maintained. Natural decay occurs as subscribers change email addresses, lose interest, or become overwhelmed by inbox volume. A typical commercial list may lose 20-30% of its engagement value per year without intervention. Decay rate is calculated by tracking the percentage of subscribers who move from active engagement tiers to dormant tiers over a defined period.
Factors That Accelerate Decay
- Low acquisition quality — subscribers acquired through incentives or purchased lists decay faster
- Infrequent sending — long gaps between sends reduce recognition and trust
- Lack of re-engagement — dormant subscribers who are never contacted remain dormant permanently
- Inconsistent content — unpredictable value causes subscribers to deprioritise your emails
- High frequency without relevance — more sends accelerate fatigue and unsubscribe rates
Best Practices
- Calculate decay rate quarterly and track it alongside net growth rate
- Set a target decay rate and investigate when it increases
- Use decay projections to justify investment in re-engagement and hygiene programmes
- Segment decay rate by acquisition channel to identify sources producing the most durable subscribers
- Treat decay management as a continuous process rather than an annual cleaning event
Related Glossary Terms
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
ARPU (Average Revenue Per User)
ARPU (Average Revenue Per User) is a metric that measures the average revenue generated per email subscriber over a specific period, used to evaluate list value and campaign effectiveness.
Attention Rate
Attention rate is the percentage of email opens that last longer than 5 seconds, distinguishing genuine reads from passive opens, preview-pane views, or Apple MPP auto-loads.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Campaign Lifted Revenue
Campaign lifted revenue is the additional revenue generated by an email campaign beyond what would have occurred without it, measured via a holdout group or statistical lift analysis.