Send-time optimisation
Send-time optimisation predicts when each subscriber is most likely to engage and delays delivery to that hour. When the prediction logic breaks, the platform may report success while subscribers receive emails at unhelpful times.
Definition
What send-time optimisation is
Send-time optimisation is a platform feature that uses historical engagement data to predict the best hour to deliver an email to each recipient. Instead of sending the entire batch at once, the platform staggers individual sends across a window. A subscriber who typically opens emails at 7am receives the message at 7am. A subscriber who opens at 9pm receives it at 9pm.
The prediction is based on past opens, clicks, or other engagement signals. If a subscriber has no history, the platform either falls back to a default time or skips optimisation for that recipient. The feature is common in marketing platforms and is often enabled at the campaign or journey step level.
How it breaks
Common failure modes
Timezone calculation error
The platform misreads a subscriber's timezone or applies the wrong offset. The email arrives hours early or late, often in the middle of the night.
Missing profile data
The subscriber record lacks engagement history or timezone information. The platform falls back to a default time that may be inappropriate for the recipient's location.
Recommendation not loading
The optimisation model fails to return a recommended time. The platform either sends immediately, delays indefinitely, or errors silently without alerting the operator.
Window expiry
The optimisation window closes before the subscriber's predicted time arrives. The email either sends at the window boundary or does not send at all.
Why the platform does not catch it
The send succeeds, the timing fails
The platform reports success when the email successfully queues for delivery. Whether it queued for the correct hour is a separate question. The SMTP handshake will complete at whatever time the platform chose. The message reaches the inbox. The platform logs it as sent. But the inbox contains an email that arrived at 3am when the subscriber is asleep, or at midday when they are unlikely to check.
I have seen a Saturday-morning newsletter that failed to send during the first hour of its optimised window. The platform reported success. The inbox stayed empty. The cause was a timezone offset error introduced when a profile field was renamed earlier that week. The field still existed, but the optimisation logic was reading the old field name and finding null values. The platform defaulted those subscribers to UTC midnight. The newsletter reached Australian inboxes at 10am on Saturday, six hours after most subscribers had finished breakfast and moved on to other tasks. The symptom was not a bounce or error. It was late arrival that looked like normal delivery in the platform's logs.
Monitoring send-time optimised journeys
How to catch timing failures from the inbox
Telltide monitors watch for expected arrival within a defined window. A test identity enrolled in the live journey receives the email the same way a real subscriber does. The monitor checks that the email arrived within the expected timeframe and that merge tags and dynamic content rendered correctly.
For send-time optimised journeys, the monitor is configured with a wider arrival window than a fixed-time send. If the expected window is 6am to 10am and the email arrives at 3am or noon, the monitor alerts. The check is receive-side. Telltide does not query the platform's optimisation model or inspect queued sends. It waits for the email to arrive in a monitored inbox and checks when it actually showed up.
Late arrival within the first 30 minutes of a window is almost always a scheduling issue, not a genuine breakage. The platform queued the send slightly late, or the receiving server delayed acceptance. Heartbeat monitoring is often the better fit for high-volume optimised sends, where the signal is the absence of any emails arriving during a period when baseline volume should be steady.
Related terms
Concepts that travel with send-time optimisation
- Triggered flow: the most common context for send-time optimisation, where recipients enter at different times with different engagement histories.
- Merge tag: often used alongside optimisation to personalise the email content as well as the timing.
- Dynamic content: conditional blocks that render based on recipient data, another common failure point when optimisation is enabled.
- Full glossary: other CRM and lifecycle email terms defined in plain language.
Monitor send-time optimised journeys from the inbox
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