What is Einstein Send Time Optimisation (STO) and how does it work?

Einstein STO analyses each subscriber's historical email engagement data (opens, clicks) to predict the time of day and day of week when that individual is most likely to engage with email. The Journey Builder Einstein STO activity holds each contact's message delivery until their predicted optimal send window.
How it works:
1. Einstein models analyse up to 90 days of engagement history per subscriber.
2. A send window is defined (e.g., deliver within the next 7 days).
3. The contact waits at the STO activity until their predicted optimal time within the window.
4. If a subscriber has insufficient history, they are sent at the default time or a random time within the window.
Requirements: Requires Einstein Engagement Scoring licence. Needs at least 6 months of send data from the same MID for meaningful predictions. Not suitable for time-sensitive transactional messages.