The Smart Sending feature allows you to limit the number of emails someone can receive from you in a given period of time. This is a good way to prevent your subscribers and/or customers from receiving too many emails if you have many active flows and campaigns.
When an email has Smart Sending enabled, Klaviyo will automatically check to see if that recipient has received an email from you within your account's Smart Sending period before it gets sent. If this person has received another email from you too recently, the current email to him/her will get skipped.
When an email is skipped due to Smart Sending, it will NOT automatically get rescheduled to send at a later time. This is because we realize most email sends are time sensitive. If you would like to resend an email that was skipped due to Smart Sending, you must do so manually (through a personal email to one person, or through a campaign for a bulk send).
How to Disable Smart Sending
You can choose to enable/disable Smart Sending on a per-campaign basis. We recommend keeping Smart Sending turned ON for most, if not all, marketing emails.
You'll see a checkbox related to enabling/disabling Smart Sending in the first step when you create a campaign.
Change the Smart Sending Timeframe
You can adjust the Smart Sending timeframe for your account by navigating to Account > Settings. Here, if you click to Settings > Email Settings you will be able to adjust your Smart Sending period within the Email Conversions & Smart Sending area.