Send-time optimization (STO) is the headline feature of every modern email platform. The pitch: instead of blasting your list at 9 AM, send each recipient the email at the moment they are most likely to open.
In our internal data across 1.2B emails, STO lifts open rate by 4.1 points on average compared to a fixed send time. Click rate by 1.3 points. Unsubscribe rate down by 0.2 points.
The catch: STO only beats a well-chosen blast time. Most teams blast at 9 AM their local time, which is rarely the global optimum. Compare STO against a blast at the global peak engagement window and the lift shrinks to about 1.5 points.
Worth using if you have it. Not worth migrating platforms for.