Send Time Optimization
Send time optimization (STO) is an AI feature that predicts the hour of day a specific recipient is most likely to engage with email based on their historical open and reply pattern, then schedules outbound messages accordingly. Vendors that offer it claim lifts, but we haven't seen an independent study we'd trust, and open-rate lifts are hard to read now that machines open so much email.
See how this shows up in your own inbox: try Outsolvi free for 14 days, no credit card.
Generic best-practice send times ("Tuesday 10am") are population averages. Real reading habits are personal: one prospect clears email at 6:30am, another at 4:45pm between meetings. Send time optimization tries to predict the right hour for each person.
How it usually works
Tools that offer it look at when a recipient has opened or replied to email before, sometimes pooling similar recipients when there's no history for that person. They pick the hour that has produced the most engagement and schedule the send for then. Vendors don't all publish their methods, and they differ in what they count as engagement.
Why opens make it harder
Most send-time models learn from opens, and opens are noisy now. Apple Mail Privacy Protection fetches images when an email arrives, whether or not anyone reads it, so an Apple Mail user's "open time" is often just their delivery time. Security scanners open email seconds after it lands. A model trained on those events learns when machines are awake. Replies are a cleaner signal, but they're rare, so there's much less to learn from.
What to do without it
A plain rule of thumb works for most B2B teams: send during the recipient's working hours, in their time zone, and avoid Friday afternoons and weekends. Then look at your own replies by hour and adjust.
Outsolvi doesn't schedule sends by predicted hour. If you want a starting point, the send time optimizer suggests a window from the recipient's time zone.
Track real engagement, not Apple MPP noise.
Start Free TrialFrequently asked questions
Does STO delay urgent sends?+
It shouldn't. Look for a tool that lets you send now when a thread is live. A reply to a warm thread shouldn't wait for a model's preferred hour.
Does this work for cold prospects?+
Only loosely. With no history for that person, a model falls back to averages across other recipients, which is close to a rule of thumb anyway.
Does Outsolvi optimize send times?+
No. Outsolvi sends when you send, and follow-ups go out after the delay you choose. The send time optimizer tool suggests a window from the recipient's time zone.
Put this concept into practice
Free tools, articles, and features on this same topic.
Related glossary terms
Want accurate tracking that handles Send Time Optimization?
Outsolvi tracks Outlook and Gmail, grades every open as a person or a machine, and flags hot leads, from $7/user/mo billed yearly. 14-day free trial, no credit card.
Start 14-Day Free TrialNate built Outsolvi after watching every email-tracking tool he had ever used lie to him about opens. Outsolvi grades every open as a person or a machine, in Outlook and Gmail, so the number on the dashboard is one a rep can actually act on.
We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.