What Is a Sending Window?
TL;DR
A sending window is the range of hours and days during which a campaign is allowed to send, set per mailbox or per campaign. It exists so messages arrive while the recipient is working, and so a mailbox spreads its volume across the day instead of releasing it all at once.
Why does a sending window matter for deliverability?
Because a mailbox that empties a day of volume in ten minutes does not behave like a person. Spreading the same messages across a working window keeps the sending pattern steady, which is the pattern warmup establishes and a burst undoes in a single campaign.
The window also bounds the damage from a mistake. A misconfigured campaign discovered in the morning has sent a fraction of its list rather than all of it, because the window would not let the rest go out until the following day.
How do you choose a sending window?
From the recipient working hours rather than your own. Field-based and shift-based roles read email at the edges of the day, before the first job and after the last, while desk-based roles clear the inbox mid-morning, so one window applied to a mixed list is wrong for part of it.
Weekends usually come out of the window, with exceptions worth checking. Some segments work Saturdays and treat a quiet inbox as the time to deal with anything that is not urgent, which makes a weekend send an advantage rather than an intrusion.
Frequently asked questions
It depends on whether the segment works then. For office roles a Saturday message sits unread until Monday and arrives in the worst inbox of the week, and for owner-operated trades the weekend is sometimes the only time anyone reads anything that is not a job.