Definition
Send time optimisation is the practice of delivering email campaigns at the moment each individual subscriber is most likely to engage. Instead of sending an entire list at a single time, STO analyses each subscriber's past behaviour — when they open, click, and convert — and schedules delivery accordingly.
STO can be rule-based (send during the subscriber's most common open hour) or AI-driven (using machine learning models that consider dozens of signals including timezone, device type, day of week, and historical engagement patterns).
How Send Time Optimisation Works
- Data collection: The ESP records the timestamp of every subscriber action: opens, clicks, purchases, and other conversions over several weeks or months.
- Pattern analysis: Each subscriber's engagement data is analysed to identify peak engagement windows. Most subscribers show consistent patterns — a person who opens emails at 8am Tuesday is likely to continue doing so.
- Individual scoring: Each subscriber receives an optimal send time score for different hours and days, ranked by historical engagement probability.
- Campaign scheduling: When a campaign is triggered, the ESP staggers sends so each subscriber receives it at their predicted optimal time. A campaign that takes 6-12 hours to fully deliver is typical.
Average Benchmarks
| Approach | Open Rate Improvement | Click Rate Improvement |
|---|---|---|
| No optimisation (batch send) | Baseline | Baseline |
| Rule-based (timezone) | +5-10% | +3-7% |
| AI-driven STO | +15-30% | +10-20% |
Important Caveats
Send time optimisation is most effective for:
- Large lists with subscribers across multiple timezones
- Broadcast or newsletter-type campaigns
- Lists with sufficient historical data (at least 30 days of engagement)
It is less effective for:
- Small lists (under 1,000 subscribers) where data is sparse
- Transactional emails (password resets, receipts) that must send immediately
- Time-sensitive campaigns (flash sales, event reminders) with fixed deadlines
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Related Glossary Terms
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Abandoned Cart Sequence Flow
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Abandoned Cart Email
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Frequently Asked Questions
Most STO algorithms require at least 30 days of engagement data and a minimum of 10 tracked opens per subscriber to calculate a reliable optimal time. Subscribers without sufficient data are typically sent at the list's aggregate optimal time or a default time based on their timezone.
Transactional emails should send immediately as they are triggered by user actions. STO is designed for marketing and broadcast campaigns where timing can be flexible.
Yes. The best implementations track the subscriber's timezone separately from their optimal send time. A subscriber in London who typically opens at 8am will receive emails at 8am BST regardless of where the sender is based.
Most major ESPs including Klaviyo, Mailchimp, HubSpot, ActiveCampaign, and Salesforce Marketing Cloud offer some form of send time optimisation. The sophistication varies from simple timezone detection to full AI-powered individual optimisation.
STO can be used alongside A/B testing — the A/B split determines which content the subscriber sees, and STO determines when they see it. However, ensure your A/B test runs long enough for the STO schedule to complete delivery to both variants.