Send time optimisation (STO) is a feature in email platforms that delivers a campaign to each subscriber at the hour they are most likely to engage, instead of sending it to the whole list at once. The platform predicts that hour from each person’s past behaviour with your emails.
How send time optimisation works
When you schedule a campaign with STO switched on, you usually pick a day rather than a time. The platform then releases the email in stages across a window, typically 24 hours, timing each delivery to the hours when that particular person has opened or clicked before.
For subscribers with plenty of history, the prediction is individual. For new subscribers, or people who rarely interact, the platform falls back on a list-wide pattern or on what works for similar audiences. Some tools offer a lighter version that tries several send times on a sample first and sends the rest at the winning hour; that is a choice made for the whole list, not per person.
A worked example: a furniture retailer in Manchester sends a Thursday newsletter. Without STO, everyone receives it at 10am. With STO, the commuter who always clicks at twenty to eight in the evening gets it shortly before then, while the shop owner who clears their inbox at 6am gets it first thing. The email is identical; only its arrival time changes.
Why it matters
An email that arrives while someone is busy sinks beneath everything that lands after it. Arriving near the top of the inbox when the reader is actually looking gives it a fairer chance of being opened and acted on. A UK list that mixes shift workers, office staff and retired customers is never well served by a single send time.
The effect is usually modest. STO moves an email up the inbox; it cannot make a dull offer interesting or rescue a message that lands in spam because of poor sender reputation. I treat it as a finishing touch once the list and the content are in good order, not as a fix for weak results.
There is also a measurement problem. Apple Mail Privacy Protection loads email images on Apple’s servers, often soon after delivery, which records an “open” whether or not anyone read the message. A platform that learns mainly from opens can therefore learn the wrong hours for a large share of your audience. Models that give more weight to clicks and purchases are more trustworthy.
Common mistakes
- Using it for time-critical messages. A flash sale ending at midnight, a webinar starting at 2pm or a same-day event reminder should reach everyone in good time. Spreading delivery across a day means some people get it after the moment has passed.
- Switching it on for a small or brand-new list. With little history the platform is guessing, and all you gain is a slower send.
- Judging it by open rate. Because opens are inflated, compare clicks, orders or enquiries between an STO send and a fixed-time send.
- Forgetting the staggered window when coordinating channels. If a social post or a price change is meant to coincide with the email, some subscribers will see the email hours later.
- Not reading how your platform defines the window. Check its help pages for how long delivery is spread over and when that period starts, so you know when the last recipients will get the email.
How to act on it
Run a fair test before committing. Split a regular campaign so half the list gets STO and half gets your usual send time, keep everything else identical, and compare click rate and revenue or enquiries per recipient. Repeat over three or four sends, because a single result can easily be noise. The principles of A/B testing apply here just as they do to subject lines or landing pages.
If STO wins consistently, use it for newsletters and evergreen content and keep fixed times for anything with a deadline. If the difference is negligible, a well-chosen fixed time is simpler to coordinate with the rest of your marketing. Either way, STO only helps emails that reach the inbox in the first place, which is a question of email deliverability.
If you are weighing up how much effort email deserves compared with paid or organic channels, that is the kind of question I answer in digital marketing strategy and consulting work.