What Is Time Zone Sending?
TL;DR
Time zone sending schedules each message against the local time of the recipient rather than the sender, so a window of 09:00 to 11:00 means those hours where the contact is. It depends on a reliable time zone on the contact record, and it falls back to a default when that value is missing or wrong.
How is a recipient time zone determined?
Best case, it is an explicit field on the contact record. Otherwise it is inferred, from the company address, a phone country code, or the region a list was filtered by, and every inference is a guess that can put a message in front of someone at three in the morning.
Large and distributed companies break the inference in a specific way. A headquarters address assigns one time zone to contacts who work in several, so a role-based list at a multinational needs the individual location rather than the company one.
What does time zone sending change in practice?
It stretches one campaign across the working day, which is useful twice over: each contact gets the intended local hour, and the sending mailbox spreads its volume instead of releasing a burst when the sender window opens.
It also changes what a daily report means. A campaign in flight has contacted the eastern part of the list and not the western part, so a result read at the end of the sender day is a result for part of the audience.
Frequently asked questions
The campaign uses a default, which is usually the sender time zone or one set per campaign. That is the case worth auditing before a launch, because a list with a large share of missing values is effectively not using time zone sending at all.