Definition
Subscriber fatigue scoring is a quantitative method for identifying subscribers who are experiencing diminishing engagement due to receiving too many emails in a given period. Fatigue is measured by tracking send frequency, recency of engagement, and engagement decline over time. A fatigue score is calculated per subscriber, often as a weighted combination of factors such as emails received in the last 7 and 30 days, days since last open or click, and the rate of decline in engagement metrics compared to the subscriber's historical baseline.
When a subscriber's fatigue score exceeds a configurable threshold, automated actions are triggered. Common actions include suppressing the subscriber from the next N sends, moving them to a lower-frequency send track, or routing them through a re-engagement sequence. Frequency capping rules set hard limits on the maximum number of emails a subscriber can receive in a given time window, regardless of fatigue score. Typical caps range from one to three emails per day and five to fifteen per week, depending on the programme's send volume and subscriber tolerance.
Best Practices
Score fatigue using a rolling window of engagement data rather than static segments. A subscriber who was highly engaged last month but has stopped opening emails this month has a different fatigue profile than someone who has never engaged. Rolling window calculations capture recent behaviour changes.
Weight recency of engagement more heavily than historical engagement. A subscriber who opened an email yesterday is less fatigued than one who last opened three weeks ago, even if the three-week-ago subscriber has more total lifetime opens. Recency is the strongest single predictor of fatigue.
Set different fatigue thresholds for different subscriber segments. Your most engaged subscribers can tolerate higher send frequency without fatigue. Use relative thresholds based on each segment's baseline engagement rates rather than absolute thresholds applied across all subscribers.
Combine fatigue scoring with preference-based frequency selection. Allow subscribers to self-select their preferred frequency during signup or in the preference centre. Override fatigue-based suppression when a subscriber has explicitly requested more frequent email, but still apply it when engagement drops.
Automate cool-down periods after high-frequency send events. If a subscriber receives six emails during a promotional event week, automatically apply a reduced-frequency period for the following one to two weeks. Cooldown prevents the cumulative fatigue effect of concentrated sends.
Related Glossary Terms
Dynamic Content
Dynamic content in email refers to content blocks that change based on subscriber data, behavior, or preferences within a single email send.
Email Frequency Engagement
Email frequency vs engagement analysis finds the optimal send frequency per subscriber. Over-sending increases unsubscribes and dormancy; under-sending diminishes brand recall. Testing methodology identifies the frequency sweet spot.
Email Interactive
Email elements that respond to user action using CSS pseudo-classes, checkbox hacks, and limited scripting alternatives.
Email Newsletter
An email newsletter is a regularly distributed email publication sent to a subscriber list, typically focused on a specific topic or theme.
Recurring Email Series
Regularly scheduled email campaigns such as daily tips, weekly digests, and monthly roundups designed to build habit and sustained engagement.
Email Survey
The use of email to distribute surveys, collect customer feedback, and analyze response data to improve products and experiences.
Frequently Asked Questions
Fatigue score is typically calculated as a weighted composite of engagement recency, send frequency, and engagement trend. A simple formula might assign points for each email received in the last 7 days, subtract points for each day since last open, and adjust for the subscriber's long-term engagement trend. Exact formulas vary by platform.
The threshold varies by programme. A common approach sets the threshold at the point where a subscriber's engagement rate drops below 50 per cent of their historical baseline or below a segment minimum. Subscribers exceeding the threshold are suppressed from the next one to three sends or moved to a reduced-frequency track.
Transactional emails such as order confirmations and password resets are typically exempt from fatigue scoring because they are triggered by subscriber actions and are expected. Fatigue scoring applies primarily to marketing and promotional sends where the sender controls the frequency.
Fatigue scores should be recalculated before each send or at least daily for automated programmes. Stale scores that are updated only weekly may allow fatigued subscribers to receive multiple emails before their score catches up to their current behaviour.
Frequency capping sets hard limits on the number of emails a subscriber can receive in a time period, regardless of engagement. Fatigue scoring uses engagement behaviour to dynamically identify subscribers who are tired of email, even if they are below the frequency cap. Both should be used together.